The XL is a great spec car! Still likely the benchmark of excellent.
The Xi is a great mod car. Always felt the wheelbase was a tick long for spec.
The Xti is the playdo car, you can mold it into anything (class) you want.
As for "goodies" to add... consider this: the cars are excellent out the wrapper. You only add tuning options, you are not adding a better or upgraded part. I won IIC with a kit car + an upper brace. No kidding, the springs and everything. There are some aluminum caster blocks that I am not fond of, they get twisted on the arm goofing the caster up. I see guys slapping every front end brace CRC makes on their cars when the conditions don't require it. If you were to be a ground floor CRC customer, I would suggest picking up a car kit, .45 & .55 front springs, a set of blue side springs to try, possibly a blue & red center spring, a set of 2.0 & 2.5mm c/f upper arm spacers, the plastic front ride height shims and a upper c/f brace. Actually I think I just rattled off everything you could possibly want. But I can not stress enough that Frank puts everything in the box that is going to cover 95% of the racing applications.