Just sayin'...
They aren't know to "grenade".. they are more known to fail for other reasons. Subaru designed the TCU / ECU to give mushy shifts. It was, after all, not meant to be a racer, it was designed to be a gentleman's GT cruiser. The factory settings gave slow, luxurious, barely noticeable shifts. "Unobtrusive" would be a good adjective to describe what Subaru aimed for. A shift kit would definitely help, as far as longevity is concerned. Limiting slippage helps keep heat and wear down to a dull roar. Excessive slippage results in excessive particulate in the transmission fluid, which results in clogged "campaign" filters, clogged passages, poor cooling and premature transmission failure. And Subaru also designed the TCU / ECU to limit engine output at WOT to "cushion" the shifts and avoid drive train shock. Some shift kits use a vacuum signal to change the resistor circuit of the transmission to raise line pressure to firm up the shifts and are simple to install and relatively inexpensive to purchase, but operate in an either "on", or "off" mode, and don't address the ECU power cut off at WOT shifts. Other shift kits, which are more expensive and require a more involved / complicated installation, progressively firm up the shifts by progressively increasing line pressure in relation to throttle pressure, and bypass the factory reduction of engine output at WOT shifts. Personally, I like the soft shifts at low throttle, and the hard snap of a full line pressure shift with no power reduction when I've got it mashed to the floor.
When I engage "warp drive", I don't want impulse power. ( yeah, I'm a geek).
Hmmm.. Break a 5MT with the torque? Define "break"..LOL.. A lot of people say they broke one, but there's a huge difference between breaking it with torque, and breaking it shifting .. or missing a shift. The SVX is a heavy car, the result of that, as far as transmissions go, is that it is brutal on the drivetrain to pull all 4 loose.. I've got an '02 WRX that makes well over 300 H.P. 250,000 on the original transmission, second clutch courtesy of Subaru due to a silent recall for shuddering. However, I don't have a performance clutch, it's the bone stock after recall upgrade. If I bang it off the rev limiter and sidestep the clutch..and I DO mean side step, not "pop".. whatta ya think happens? Well, it slips!! It leaves like I got ass ended by a semi, the tach stays at 5K until it hooks up, rips to 7,250 RPM.. I leave my foot on the floor, bang it into second, side step the clutch again,and my accellerometer says I hit 60 MPH in 4.9 seconds.. And when I come to a stop... yes.. I can smell clutch. I love the smell of over heated organics in the morning!! I've only done that to her three times. Once to see if I could hit 60 MPH in less than 5 seconds, and twice for money. The factory clutch is designed to slip under that level of abuse, rather than strip output shaft splines or bust transmission gear teeth. Subaru designed them like that to avoid warrantee claims.. Bust a gear? might be their fault... Smoke the clutch... It's definitely on you.. A 5 MT might be a bit more fragile than a 6 MT, but enough abuse will still bust parts no matter what.. It's just a matter of when. Hot tires, hot pavement, 320 HP and a grabby clutch, with 3,500 pounds pressing the rubber to the road.... Sooner or later.. Bang! Wooot, woot, wooot bang! Crap! I need a tow truck,..sucks!
I don't know enough about the valve body of a 4EAT to give you a definitive opinion, but I'm not sure if manually shifting while backing off the throttle is best or not. The transmission has accumulators and springs and valves and reservoirs and the TCU talks to the ECU, which talks to the TPS... Etc, etc... If backing off the throttle lowers the line pressure, it might be better to let the trans do it's thing, especially if you toss a shift kit in. Some searching of this forum would probably find a thread about that, especially as I know there's a mixed school of thought about leaving the shifter in third gear around town.. Also, make sure the brake band is set properly. If you've got a sloppy 2 / 3 shift, be sure to check the setting.
Keep an eye on the ATF temp gauge? Do you have a stand alone gauge, or are you referring to the idiot light on the dash? If you're going by the light on the dash, by the time it's lighting up, it might be a bit late. You've got a "pretty large cooler".. As long as it's properly mounted where it gets tons of air flow, overheating shouldn't be an issue.
No clue about welding the center diff.. did you mean with the manual transmission? The USDM 4EAT is 90 /10 until the front wheels slip, then it splits power up to 50 /50 .. There are cheap, almost free mods that would let you pre select that electrically, so that 4WD is engaged before you mash the happy pedal. Those kinda mods are just a search away. they're quick too.
Then... there's the "Elephant in the room".. Why the manual transmission? Granted, some people just don't feel like they're driving unless they're driving a stick. If that's the case, then have at it! But, If it's a matter of performance, reliability, cost...have you considered it'd probably be half the cost for a 4.44 swap? A 4.44 trans, 320 WHP..you'd be looking at low 13s' in the 1/4 mile... all day every day. Personally, I love the stock gearing of the SVX, the low revs on the highway. It's my cruiser, that turns heads and still will smoke a lot of stuff, especially from 50 to 80 MPH.
My WRX is my bantam weight blitzer. But hey!.. The best part of having your own toys is getting to do what you want with them!
Post back what you decide..
Good luck, have fun..