I wouldn't want it any further back if I'm honest, where it is, the gear lever will come out just in front of the seats which is nice.
As for the weight difference I have no idea but it's sitting very high at the moment and if it's more than 5mm I will have to have some springs made. It's a bit early to tell yet as the gearbox isn't in, just the engine on it's own. The mx5 engine is all alloy bar the block I believe I just don't have anything to weigh the two on so I will be going on old ride height vs new ride height.
Robert, I didn't want to go too mad on power increase as I want to keep the economy and avoid snapping drive shafts! however I'm sweet on trying out nitrous
About half the price of a turbo kit and something different to try (my last car was a mk3 Toyota supra 3.0 turbo)