on my 2003 mk4 tdi arl golf, just above the oil filler cap, there is an unused plastic connector coming out of a short pipe between head and the manifold, what is this for?

also i seem to have an issue with low boost! no smoke whatsoever but had it remapped few months back, yet recently a dyno showed 145 bhp! it has just had a new n75 valve. It struggles to go beyond 3000rpm when on the move, when it does, nothing really happens! I can't really see any leaks in the boost pipes, normally they would be dripping oil if there was! my intercooler is probably the original but i dont want to spend the time/money changing it if its not the cause! my turbo does seem to spool by itself after short journeys, as if i was revving the engine when im not. Almost sounds like the fans are kicking in but its coming from turbo, car has 165k. 'economy' remap few months back should mean i see 170+ bhp, not 145. also i struggle to get more than 50 mpg on motorway, what could this be?