Where exactly are these modded cars faster than yours? Are you getting pulled on straights? Are you being outbraked? Are you being blown by in the corners?

You have to be more specific as to how exactly you're being outperformed.

Let's also talk about how experienced you are compared to these other drivers. Are they on your same level? Do they know the track much more? I don't roadrace cars, but I do race motorcycles and know that a slower machine can be ridden much faster than a more powerful bike off-road and on a road course. It's all about the driver in most instances, unless you're just grossly underpowered.