That turbo you posted above is a vg unit I think, I cant find any data against the part number thats in the listing.
The GT17 is deffinitly bigger than a K03, and for sure a bit smaller than mine lol. My mate fitted a GT17 off a SAAB to his mini and it was great, pulled well from low revs. I think he dyno'd it at 18psi with 147bhp with 176lbs/ft of torque (@ mikanicks or whatever its called) But you have to remember thats on a mini with only 2 tiny inlet ports and 3 exhausts. Standard head as well, hence the low power high torque.
Personally I wouldnt advise a BVH. For the money it just seems like such a small gain, the VW exhaust valves arent exactly tiny as it is. I dont know how much they are but im sure you could put that money to good use somewhere else and then fit a BVH later on if you really need more power. Im guessing a GT cam in a BVH will probably only flow as much as a lairyer cam in a standard head anyway, rendering a BVH with a GT cam a bit pointless.
I adjust the boost with a bleed valve (or by shimming the actuator if its miles out), its a bit crude but it does the job well enough for my car.
200bhp is a massive amount of power in a polo though and I think its pretty pointless, I guess for a lot of people its just a target to work to or its purely for bragging.