No answer can be universally valid across the entire U.S., so you’ll have to tell us where you are located, where you race, and whether you intend to compete on a local, regional, or national scale.

To answer your question, IAME motors were designed from the ground up to be kart racing motors, so they don’t have nearly the room for improvement through blueprinting that the KT-100 has.  You can run competitively right out of the box, and a lot of people do.  Certainly at a local-and-occassional-regional level.

Not sure why you can’t run the PRD where ever you plan to run a Leopard.  The PRD is basically a Leopard clone, and I’ve watched decent drivers in PRD’s have good local success against Leopards.

The Leopard is being phased out so if you are buying new, the X-30 is the way to go.  Neither should be more expensive to run than a PRD.  The X-30 should have a longer rebuild cycle than the Leopard (30 hours versus 20, depending on how hard you drive).

However, there are a couple of new Leopards on sale here: http://www.italianmotorsusa.com/products/leopard-my09

I would vote x30 from what i know (never driven one). I base this off that the prd has insane vibration and the x30 is supposed to be much smoother. The x30 and TaG is strong in most regional series.