You can filter by any combination of PP, weight, power and drivetrain (in addition to tire limitation, disallowing tuning & nitrous).
OR you can filter by country or a group of up to 5 makes.
You were halfway there, PD. Combining the old performance restrictions is a dramatic improvement; being able to limit car choices is a nice addition. Not letting you do both seems needlessly, frustratingly limiting. I hope this gets fixed eventually, so much potential..
Also, even in GT5 there were some great, balanced and diverse races. It's a huge chore to find all of the cars that compete at a given level, but if you start with a car or concept in mind it's usually possible to build a balanced field around it.