Looks like no one wants to answer this one so I will take a stab at it.
Basically the answer to your question is.....no differance. The early
cars had lighter flywheels, higher compression, and a sigle branch
exhaust to name a few. The later cars had lower compression, heavier
flywheel, dual branch exhaust and a better camshaft profile. Even with
the smog equipment the later TR-6's had a tad more horsepower. But most
people will tell you that it is hard to tell any differance between the
The early 2.5 liters had a better crankshaft, but the later had a better
block. So you can see if you mix and match you can come up with a real
nice combination of parts to make the 2.5 a better performer.
Thats about it without getting overly technical.
Pat in NC