You have to think of an LS swap as a way to change the character of a car and replace an OE engine that has too many issues to contend with. It's NOT a "cheap upgrade" project.
Take the 996 911. Guys think an IMS nuked car is a perfect candidate for an LS swap. Sure, if you're ready to spend MORE than going with an IMS-solved 911 engine. By the time you buy a solid LS and sort out all the issues with the swap (and yes, the kits all require a LOT of work to install) you're not saving a dime, and you're probably out a bunch more dough.
I love the FD RX-7, agree it's one of the best looking cars to every be loaded on a boat from Japan, and in many ways it's a prime LS swap candidate. The LS is compact and light, keeping the RX from gaining too much weight. But, don't think it's a way to fix a broken RX-7 on a budget. It will cost a lot.