Just to add one thing to the cons list, which applies more to an engine swap than a TT/SC kit, and that's to write off the very vast majority of the cost of the whole shebang. A TT/SC'd car is relatively easy to put back to stock and if you buy second hand (which, it has to be said, most people do these days) then you'll reclaim most of what you spent. The time & cost of putting an LS'd Zed back to stock is going to be horrendous unless you have the ability to completely DIY it, and if you leave it all in then you'll never reclaim anything of what you spent when selling the car on. Well, maybe a grand or two, but certainly nothing substantial.
That said, this was the only mod I ever gave serious consideration to doing when I had my Zed. Crate engine was approx £10K, I figured another £4K for bits and then another £2K on top for a garage to finish off the bits I couldn't, plus 10% just in case. Trouble was that would then have left me with a car that would've been nigh-on unsellable not to mention doing all the other bits you really should do for 400bhp+ (suspension, brakes etc) and my car was already well modified in that aspect at a serious cost. You have to really love your Zed, want to keep your Zed forever and/or have a stupid amount of money to burn before considering this IMHO, as you'd be far better off just buying a quicker car in the first place (TVR if you want the V8 sound + speed, 996TT if you want the speed + luxury).
And despite all that, I would love for someone to give it a go over here as I think it's a far more interesting project than just slapping a couple of turbos to a car.