What Tom says. 1-3/4 to 2" is fine for the pre-cat pipes on a stock SVX, merging into a 2.5" mid-pipe. The smaller tubing also makes routing easier, as the bends are tighter.
Here's what I did with mine. I haven't done a sound clip, but it sounds good.
http://www.subaru-svx.net/forum/showthread.php?t=51867
I'll probably sell this one in a year or so and build a new version in stainless, with a resonator and flanges or v-bands all around. Need to get a MIG for myself first. I have a flux core welder, but it's a pile.