I've turned 2:45's in Jeff's Sentra on street tires, so a proper sports car on race tires should easily be in the mid-low 2:30's. Best way to answer your question and give yourself a legitimate "target time" would be to have someone like Chuck take a few laps in your car. Someone who knows how to get a "fast" RWD sports car around the track will be able to get about 90% out of your car in just a few laps. Ultimately, you should be a little faster than that, but that will give you a good idea of where you should be.

As for brakes, unless they're consistently overheating, don't assume that they are "inadequate". (and, of course, you can get feedback on your setup by having an experienced driver take your car out)