Re: my new diff...
ROFL, sounds like me!!!
I got a GT-P BA FPV and changed the centre from 3.46 to 3.73 (AUTO) and yes the speedo is running off the ABS sensors, meaning you could shove 4.11's in there and ur speedo would be correct, but change the rolling diameter of your tyres and it would be out.
this doesnt help your prob with the changing of gears, well My GT-P does the same thing, in first it will accelerate untill it hits the revlimiter then about a second it changes gears, but each gear sits on the rev limit a bit longer, i have a tiptronic Auto box, so if i want to push the car hard i use that, there is no fix atm.
I believe the only way to fix this problem (what i have been told) is to take it to a transmittion place (one that u really trust) they drill out the valve to get the trans to shift quicker (shift kit) no one has had this done that i know of and i dont know if it truly works, I havn't had this done because i simply dont trust the Apprentice Mechanic with my car!!!
if ya wanna chat more pm me ya number, what state u in?
hope this helps :(
just my oppinion herrod wouldn't have a clue, but if ya get anything outa them let me know...