Not necessarily true. A hybrid turbo with variable geometry can easily produce a nearly lagless boost without dropping the top end (eg, VNT 15/17 and VNT 17/20 hybrids). A boost valve made from common brass fitting can be installed between your vacuum powered actuator and intake to reduce overboosting beyond requested during hard acceleration also helps to reduce lag and protect your investment (I made one and it works great!).
Are you referring to the blow off valve, or the wastegate. Either way I'm more than familiar with them. I have been wrenching on my WRX since 2002. A well tuned electronic boost controller will actually allow as fast and safer spool up than a manual one.
Are you a VW guy by any chance? TDi's use the VNT turbos is why I ask. Doing it yourself is one thing, but the OP asked for a kit. Is there anyone out there offering a variable geometry turbo kit with management?
edit: just read your sig, you are a VW guy.
If the OP is set on a turbo, then properly sizing the turbine and compressor housings and wheels is a much more realistic solution.
He's talking about a manual boost controller. Just, FYI.
As stated, the turbo will not enjoy itself in muddy/wet/extreme conditions. The bearings, etc. are all things that can/could fail if exposed to certain types of wheeling.
Your biggest limiting factor would be the stock motor. It's an inline-6, which means the main journals are somewhat beefier than a stock v6/v8. However, it still has its limits. The kind of power boost you would get, would be minimal at best (maybe 100hp at the most... and that's being extremely positive) and it wouldn't be at the part of the power curve where a jeep would need it most or it might end up being an on-off switch that'll snap axle components.
Aside from that... You could end up with lag, but on a motor with the behavior that ours has, that is highly doubtful. Sized right, the 4.0L will spool a turbo efficiently. If need be.
You do not really want a turbo for this type of vehicle. It can be done... but it's pretty much a throwing a hot-dog down a hall-way kind of situation. I would stick to gearing correctly and at the most, using a stroker. Power adders on a Jeep would be like a midget wearing platform shoes.