Not related to this question directly, but from previous trackcars I would say enjoy the "evolution" of your car.
I really enjoyed my 200sx as I slowly evolved it, it was fun doing that journey, but with my E36 track car I kinda of did everything in one go and didn't enjoy it as much (also you may change your journey as you go along).
I would say some of the biggest things were:
* Bucket seats - they give you a sense of so much more control as you're not sliding about
* Good brakes - again as above gives you a sense of control that they won't overheat etc
* Tyres - you said you got "decent", what ones? A great track tyre will make a good difference (once you get the hot tyre pressures right)
* Suspension - adjustable helps fine tune for different tracks but of course adds more complexity (also this includes stuff like polybushed engine mounts and suspension)
* Losing interior bits - making it of course lighter, but if you use the car a lot will get annoying (sounds like this won't be a problem for you), also you get a sense of more speed due to extra sense of speed due to extra noise and vibration etc.
I would say the above will be a good starting point and you'll be exploiting more of your power / getting faster, you may then find out that you won't want to go meg engine, or the literally opposite, as I said before you will come to different crossroads.
Also if you did the above then decide about engine, it won't take away from what engine you decide, as you'd probably want to do those bits anyways (as they are "cheaper" than the meg conversion).
Sorry it isn't specific to Clios as i've not tracked it (so
@Beany and
@Big Ben reviews will be more specific) this is just a more general track evolution explanation.