The bigeest improvement and difference will always begin with the "nut-behind-the-wheel" (the driver). Next, is the tires, and work your way inwards... but it eventually becomes an overall package of changes that'll make the final differences. Physics is the final law.. so.. choose your base platform wisely to begin with and plan carefully looking into your future wants/needs, etc. In other words, just don't spend a ton of $$$ and expect the best... consider improving your driving skills (track time = time and $$, driver's schools, etc) as part of the overall plan.

Excluding the variable club rules, I believe all of the XV setups were well designed for handling setups, especially their frame strengtheners (as their videos display). Personally, I don't have experience with the XV items... I prefer simple bolt-on type of mods. XV has kept some of their specs (TBs and leafspring rates) closed from anyone knowing what they are.