What you need to ask yourself is this.....
1. how much further do you think you can push the C1j ?
2. If you did put an F7 engine in your 5, could you be bothered with all that hassle again, finding conversion parts, wiring etc. etc. As per PM, it can be cheap if you get it right, but if you go forged etc with bespoke stuff like Robbie it will be a money pit, but it's a better engine to mod for more power...
I know I love the GT Turbo but if I did things again I think I would sell up & buy a valver or willy to convert, so much easier & the willy has widetrack as standard...