I stuck with the Renault unit through many of my adventures with uprated Lotus engines, and never broke one. I still have a couple of them lying around somewhere. It's actually a very strong box given its original application. It was raced, and is still used in many uprated Esprits, as well as GT40 replicas etc.
However, engine power has marched on significantly, but I believe the old Derek Bell re-engineering of the UN1 internals is still available through GTO Engineering
If added to the cryo-tempering and REM isotropic super finishing procedures becoming more available lately, I think it could easily handle any uprated Lotus Esprit engine - except perhaps the most extreme ones.