just an added thought, didn't the Esprit go with Citroen gearbox/transaxles when Lotus wanted more power for the heavier cars ? If so, it might be worth checking out Esprit forums.
The Giugiaro Esprits where fitted with the Citroen SM gearbox, as the power increased and with the introduction of the Turbo Esprit even this gearbox struggled to cope - especially due to Citroens lack of development and eventually their desire to end its manufacture.
The 'Stevens' Esprits used various versions of the Renault UN1 gearbox, not just because the Citroen box went out of production but also because it was capable of coping with higher outputs from the outset. Renault continued to develop this over the production life of the Esprit pretty much (versions were fitted to the Renault Alpines which helped spread development costs for higher output rated vehicles) and they were still being used in the V8 Esprits which had 350bhp and 295lbs/ft - which was actually outside the rated capabilities of the gearbox!
The UN1 gearbox (various variants fitted to a wide range of cars and vans) is also a popular choice for the likes of GT40 replicas which usually have V8's fitted - forums relating to GT40 replicas are a good source of info tbh.
There is/was a company based in the U.K. Called GTO Engineering that developed an uprated set of internals to cope with higher outputs, they had various options available which depended on what you required but the key weak areas had been addressed (input shaft was uprated and made as one piece along with improving 5th gear - main improvement was to stop it making its way out of the casing when rolling on and off the throttle under heavy load!) - from memory I'm pretty sure they had a 6 speed option available too....? I know of a few people with the GTO kit installed in 500+bhp cars and they have survived pretty well for what is a fairly cheap option (when compared to a Hewland for example!).
Hope some of the above is of interest/helpful?!
