Well, lets see. I took up autocrossing last year in a MKIV so I oughta be able to add something.
I’d recommend not modding it anymore. For autocrossing at least, the car will be better than you for your first season. If you don’t believe it run a few events then ask one of the Potocki’s to drive your car for you for one run. I’ve got $20 that says they’ll be at least a couple of seconds faster than you. Also, this will give you time to learn what you don’t like about your car before you start throwing money at it based on internet banter.
My suspension was totally stock worn out 100k mile pieces and I still managed to turn in faster times than quite a few “faster cars.” On the flip side there were tons of faster drivers that turned in way better times than me regardless of what they were driving. Wait till you get pwn3d by the chick 2 footing an automatic Beatle! 
After my first race I changed brake fluid and got a set of decent pads. I didn’t pony up for them but I should have gotten Hawk HPS or HP+ pads. If you don’t have decent summer tires, get some. I used Khumo Ecsta SPT’s and they worked decently and were cheap.
I replaced the frame-side dogbone mount bushings with polyurethane ones and it cut down on wheel hop quite a bit.
All autocross though. I know nothing about track days.
Oh, and one of the few really really good things about MKIV’s is that they’re one of the safest cars on the road. Don’t ruin it by ditching your airbag.