I have been considering a LS1 swap for some time now. I have considered z's, 951's (944 turbos) and RX-7's. I have also driven numerous 911's and 930's. For me, when considering swaps, the question came down to whether the swap improved the car. A 930 turbo has a race motor, stock. It is simply a better motor than an LS1 (which is an amazing motor in its own right, it just is not as 'track ready as a Porsche 3.3 turbo) and can easily be made to make equal or more power. So with a 930, stick with stock since making the swap is not cost effective (even with the Porsche tax) and you won't necessarily end up with a better car.
However, with 951's and RX-7's, the stock motors are ticking time bombs and will cost serious & to make significant power upgrades. Where you can do a swap, maintain or improve handling, increase hp and increase reliability, why not? An LS1 in either of these car simply fits the bill.
Z's only make sense because it costs mucho $ to get big hp out of the stock motors. Stock z motors are fantastic for what they are, so with z's I think it boils down to what your ultimate goal is. An LS1 won't necessarily make your car 'better', just faster.