Hello and welcome to the site mate
what engine ? there are quite a few 1.8t varients from 150 up to 240 in standard tune
what dyno if thats just got a remap on it, 236 is starting to get near of the common turbo used on them and requires alot more work that just a remap, things you would hear and see
to get 350 isnt hard but can be expensive depending on what you class as expensive.
doing it yourself will take a while unless you can buy the parts off someone who has givin up
going through a tuning company your looking at around £3.5k
but really what is the point ? the mk4 golf is a barge on wheels and FWD. trying to get 350bhp down isnt easy through the front unless youve spent money on
a) tyres
b) the clutch, DMF, and diff
uprated clutch and DMF is just over £1k for parts
diff is about the same
then fitting if your not doing it yourself
so you could quite easily have spend £6.5k, allowing for some other things, to get a 350bhp FWD car.
if your wanting to go faster but keep the 1.8 engine then look to swapping to a audi S3. about 90% the same as your car however has haldex 4 wheel drive and comes with 225 to start with. a remap on this will take you to around 260-270 bhp and if you start looking into hybrid turbos then 300
the 4 motion chasis is also apparently handles alot better than the FWD version shame VW only used it in the V6 models