most have been answered above.  
1-driven gently MPG will improve, driven hard it will be worse
2-Driveshafts - Never heard of any one have an issue with drivveshafts, and people have had these pushing out 500+Hp.
3-there are so many variables here.  Driving style, tyre brand/type, you wil get more wheel spin if you use the accelerator as an on/off switch.  you get used to the power very quickly and hoow to best put it down.
4-no other parts need to be replaced.  the only common fauklt is the original diverter valves used to tear.  VW released a new piston version which solves this and is only £48.
5-brakes-depends how you use them.  road upto around 260hp, you should be ok.  For track, you need to look into them.
6-Chassis upgrades will help massively in hgetting the power down and are highly reccomended for high power cars.  a 240Hp stage 1 GTI will be 100% fine on oem suspension and brakes
7-In the wet--irrelevant of power, these are easy to spin up in the wet.  with a remap, you will find the torque is that much improved you can use 1 gear higher and still get the same rate of acceleration as standard.  Driving in the wet is frustrating in a FWD full stop.  you just have to learn to drive around its weaknesses.  good tyres are a must