Each person would have his ultimate Spec & it will always be dictated by cost thats the way it is. The more you spend the better you can make it.
The thread starter talks about taking the engine to the absolute maximum is that for one race or 40k thats what you have to decide. You can have over 1000bhp on methanol from your YB but it will cost 50k & be akin to a grenade. If you want 40k miles then you ultimate Spec will start with a 4.

The higher the maximum Power The more things need to change to cope- Special gearboxes, 10" diffs, rear beam, chassis connectors, coilovers, uprated prop/drivesghafts, Bigger brakes, Coilovers. Its easy to sink 70k if you mod everything. If you have a 20k budget you have to pick & choose your bits with care but to you it can still be the ultimate Spec thats what makes this hobby so great.

Must admit im working on a car at the moment that i would love to own.
Mk1 Fiesta with a 500bhp YB engine, rwd & incredible body mods to get huge wheels . Its not got huge Power but i would love it & im sure its the owners ultimate Spec.