But that is what forums are all about. If you don't want to discuss something don't mention it in the first place, expecially performance claims
It is physically and mathematically impossible for a 430kg (which you haven't explained how you achieved) car with 90bhp and two people in it to achieve 0-60 in 5.4 seconds. You are looking at around 7 seconds and just under 16 for the 1/4 mile.
To achieve 5.4 ypu will need around 140bhp and 115lb/ft, good tyres and plenty of launch practice.
I imagine that the people who have been in your car are of a similar age to you and haven't experienced much in the way of performance cars, so a lightened and tuned Fiat would, indeed, seem fast to them as it is all relative to your personal experience.
160bhp/tonne is good but not earth shattering. My Robin Hood kit car has around 140bhp which equates to 200bhp/tonne and runs low 15 sec quarters.
My friend's Ford Pop has a 550bhp Ford V8 in it giving it 550bhp/tonne. It's when you get to this level of power that new pants may be required
With regard to tuning the Megane engine, I know nothing about them. However, if it is a 4 cylinder OHC lump, then tuning this is no different to any other engine of a similar size and configuration. Before embarking on a rebuild, I would give the engine a full service and tune to make sure it is OK and working to its design spec.
If, after doing this, you are not happy with the performance then don't bother with cut back exhaust systems, cone air filters etc on their own, as the increase in power will be marginal and you are after, I guess, a decent hike in power.
A 10% improvement can usually be gained by a simple remap, cost around £250. IMO a decent exhaust and a less restrictive induction system would then allow a remap to take advantage of the engine's existing potential and increase this to around 15%. Standard power is around 111bhp so a remap/exhaust/intake would boost this to approx 126bhp.
A cheap way to get extra power for when you want is to fit a ntrous oxide system. Looks flash and a 25-35bhp system is usually safe when using standard engine parts. Once you improve the engine's stength you can increase the nitrous power by a simple(ish) jet change
If you want more, then engine mods are required (head porting, performance cams, increase in capacity, etc) and the limit is how much you are prepared to spend.
Adding a turbo to a non-turbo engine is expensive, so a better bet would be to get a turbo engine to start with and work on that. Remapping a turbo engine will increase power by 25% on average.