Thanks. I wanted to help out everyone who was clueless about TDUPE (as I was before) and at the same time help me remember the settings better.
BTRQ is a file inside the car's exterior BNK (i.e. SKYGTR.bnk) which contains information about the car. I'm not entirely sure as to what it contains, but am certain that one of which is the redline limit. For example, let's take an Audi TT which has a rev limiter of 6500 rpm. Say, you decided to replace it with a Ferrari 458 Italia that revs up to 9000 rpm. Naturally, you'd edit the physics so as to make the TT (now an Italia) have a 9000 rpm redline. However, once you do drive the car, you would notice that it would rev properly to about 6500-7000 rpm properly, and past that, it would rev very slowly and when you actually reach 9000 it doesn't have the "bouncing off" effect on the limit but rather just stays there. This is because the BTRQ of the TT BNK was unchanged and still limited the car to 6500 rpm.
I'm not particularly sure, but what I did in a situation like that (forgot which car, CLS55 I think which I replaced with C63), was to replace the BTRQ of the car with the BTRQ of another car that I knew had a much higher RPM limit. :)