Coz, I am no expert on chassis engineering, but I have found more caster makes the car a lot more stable at high speed, and stops it darting around. It also dials in some self centring. The steering may have become a bit heavier at parking speeds, I have not noticed. Your GT5 would benefit from this mod, because you have excessive scrub radius. When De Tomaso came out with the GT5 they just added 2" to the outside of the wheel. What they should have done is put an inch either side, and made longer A arms. They corrected this on the SI in 1990.
But of coarse, those gorgeous GT5 wheels wouldn't look so cool without there positive offset.
Djezc, I charge 200 pounds for this mod in England. I don't do an exchange because some are in bad shape. People normally just send me there arms. I would be happy to do them for you, but as you are in the US, it would probably be cheaper for you to send them to Pat Mical in Boston.