Hmmmm, well loose subframe bolts isn't going to help matters. Well done VW!
Pulling to one side only under power is usually movement in one wishbone as mentioned already, or severely worn engine/gearbox mount(s). I would let VW put it right seeing as they did half a job last time.
Is this not just torque steer? I thought it was a common thing in high-ish hp fwd cars and was to be expected. my car does this as well and I thought it was strange at first but have got used to it after reading about it.
Not in the gears the OP mentioned. MK5s don't really torque steer much. What you're feeling could be tramlining under power, which is different to TS. On a completely flat road, mine doesn't TS at all. But if I accelerate hard in 2nd or 3rd on a lorry rutted road, it pulls all over the place.