If you actually understand the physics behind turbos, then you'll understand this:
1. turbo lag is dependent on the size of the turbo & exhaust system - turbos are turned by exhaust backpressure...so on a 4.0L the turbo only needs to be active between idle-5,000 rpms, thats a lot different than a race car where you'd want 4,000-9,000 rpms.
2. turbo do not have a parasitic loss like superchargers.
3. boost can be talored to engine components, on the fly with adjustable valves
4. a correctly sized turbo will produce more fluid power and torque than a conventional supercharger. i'm excluding electronically controlled variable pitch turbos, and the fancy electronically controlled mercedes superchargers...just talking basic forced induction here.
505 Performance makes the only commerically available turbo kit for a 4.0L. www.505performance.com
I have ridden in a Rubicon with a 505 Performance kit installed. it was silly fast for a jeep, even kinda scary considering the SWB isnt that stable to begin with. i have driven and ridden in many fast cars, from supercharged mercedes, V8 caddy's, AWD Subi's, Saab's to a WS6 Firebird, so i know what stability at high speed is...and the Jeep doesn't have anything close.