I can't call it a scientific fact, but my experience is that - yes, car size does make a difference. At the stereo contests and such a small car such as an old Honda CRX can hit much higher SPL numbers than say a Suburban, and do it with less speaker area and less power. I've seen it a million times, and it is widely accepted as a fact though I can't give up any formulas to show it. All that goes out the window (literally) with the windows down though since the car is no longer an enclosure.

I'm stumped on the other half of the question - it's a good one for sure (how to tell if the amp is running out of steam). I'm trying to think of ways to measure it - dimming headlights, or maybe with some kind of datalogger to see if it's not getting the juice it needs. The cap you have should take care of that though (old school rule of thumb was 1F per 1,000W).

Gut feel says that the amp is plenty, but you know how that goes.