Do you plan to play other games on Xbox Live? If not then definitely do not pay $60 a year just to play this game online - the DLC and online races are not worth $12 a month to me. I'm a subscriber so I can play Gears of War and Space Marine online with my RL friends who've moved away.
I have since loosened my rear sway bar back up a bit, the lift oversteer was just too much on some turns. This game doesn't treat FWD understeer as badly as the Gran Turismo series so I'm learning to take a smoother line and set up my brake points better rather than just sliding the rear end around and using torque steer to straighten out. That trick worked well in GT3 and 4 but here I find myself getting pointed in odd directions at unfortunate times.
Brakes, tires and suspension are worth more early on for cars IMO. They also typically don't push your car into another bracket as quick as power mods do.
*Edit: Aaaaand I'm no longer driving the GTi. I got to pick a Ferrari GTO or Countach in the "80's Pinup" category and took the GTO. Hole. Eee. Shit. I badly needed the front splitter and rear spoiler for downforce, uped the tires to 17's, added more weight back on, lowered the suspension, and loosened the front springs a bit. The car is still right at an A600 rating and it is so much fun. I spent an hour carving up the 'Ring and I'm having a blast! The car even sounds like a Ferrari - it just screams!