Never mind whether a modded Mk5 is better than a standard Mk5 day to day..
1) Do you enjoy driving your car ?
2) Do you have any problems with your car that are going to cost you a lot?
a) A problem that needs fixing right now?
b) A big problem that you can't seem to fix?
c) An issue that you would like to fix but isn't essential to the running of the car?
3) Do you need a car for everyday duties? (Commuting, work, driving kids around, etc)
4) Do you need a different type of car?
a) smaller for lower running costs (insurance/tax/finance)?
b) bigger for lugging stuff/people around ?
c) more efficient car for cheaper long journeys ?
d) safer car for the kids? (ie if you had a Mk2 and wanted ABS/Airbags/etc)
These are the sort of questions I had to consider when looking at changing cars 5 years ago. I had a lovely Mk2 16v GTI, loved driving it, but my finances were in the crapper having just moved and living costs gone up and it needed a little work which I couldn't afford. So I sold it to make my finances slightly better and reduced my insurance by only driving a little Polo as a run around.
A year or so ago I was driving a Mk3 CL, nothing special and nothing major wrong with it, but being a more basic model it didn't have much in the way of safety features and as we had a new baby wanted a car that was going to be safer, plus we'd realised that we needed a bit more space for buggies and things. So we found the money to upgrade to an Audi A4 Avant - more space, more safety features but still within our modest budget.
I enjoy driving the Audi, it may be bigger, but it's still an enjoyable drive. I will be trying to keep this for a while and even if it costs a bit in maintenance, it'll be worth doing.
So, T_L_G, what's "wrong" with your car ? If there's nothing actually wrong with your car and it's just a case of not being able to afford the mods you would-like-to-do, can't you just enjoy your car?
Or are you bored with it? Does driving it not bring any pleasure?
If you really can't afford work that "needs" to be done or the car doesn't interest you anymore, sure sell it, get something else. But you never know, that car could end up costing you lots. Isn't better the devil you know?
