Yeah, new cars will have a small advantage. It is small but there and people will try to make ground there. I am guilty of this as well. But at least I know it is the chassis and not an ESC/motor thing.
However, more recent trends in chassis development and technology have produced a car that isn't necessarily faster, but one that handles changing track conditions and have larger setup windows.
To combat this one has to really work with what they have to keep up Witt the current trends. Flexy chassis seem to be the norm...if I had an older chassis I would look for ways to emulate this. In a tc4 for example, you can grind off the chassis support brackets to get some chassis flex.
Bottom line, yes, chassis changes are hard to keep up with. If you have an older chassis It can still win if you pour some love and time into it. Which has always been the charm of this class in my opinion. It is a driving and setup class, not a buy the latest and greatest class. We have guys running tc4's and kicking butt. Really ticks off the guys that went out and dropped big money on tc6's and t4's!
The big issue at our track is inconsistency in motors. Not all 25.5 ballistics are the same. With an FDR limit you can only make your car faster to a point. For example, we have a 6.9 limit. Everyone runs max timing and on a simple oval track there are drivers that are a full second off the pace! Not a setup/driving issue as we swapped motors and the times switched. That will cause racers in out club huge frustrations and they will go out and get a new motor and start the chase the leader game with their wallets.