Seeing as you have already spent your hard earned and you must know the car like the back of your hand then I would keep it if. Bang for ya buck nothing can beat it. But just because you have done lots of work, doesn't mean to say the spending will stop, and could very likely continue...
I've learnt the hard way and settled down over the years, but it took some real soul searching and a priority realilty check before I came to my senses. What really clinched it for me, was do you really need a car with anything over 280bhp and circa 1300kg on UK roads. ?. Its nice to have bonkers power but it comes at a cost, and in truth I could cover ground in my 300bhp classic just as fast as its predecessor 400+bhp blob, all I have to do is just push the car and my bottle a little bit more.
And even if your into track days - For me presonally I have had just as much fun if not more in a £1000 pug 306gti round a track. I might not put in the same lap time, but I will have had to work the car within and inch of its life, but I'm happy to do this in the knowledge if I break it so what.!.. If you push a highly tuned car within an inch of its life you will either crash it, or be constantly worried about breaking it, or actually break it..
Heavily modified cars are money pits, and they seem to be an endless chase for upgrades. As soon as one part is near right, then then another part say transmission gives way, and then breaks, then suspension and so on..... And they are like pringles...once you start you can't stop
A Stock STI whether its a classic, blob or bug is a very well balanced package and reliable, and apart from some subbtle mods should be left alone IMHO.