You'll have nothing to gain and a lot to lose by changing to synthetic. DOT1,2,3 &4 are not compatible with DOT 5 (synthetic.) This means that every rubber part in the system must first be replaced and all the remaining steel parts thoroughly flushed. Yes, there are rubber parts in the ABS pump, so plan on either deleting it from the system or buying a brand new one. Then again, anyone that needs DOT5 wouldn't want an ABS system as repeated testing has shown over and over that ABS systems don't/won't stop as fast as non-ABS systems.
BTW, not to pick nits but the proper word is 'hygroscopic'.
As I recall DOT5 is even more susceptible to moisture than DOT4. You may want to check into that, I have been wrong once or twice before.
