the wastegate governs the level of boost your turbo suplies on a 300 tdi i think that its set to 13 psi or around that on a td5 i dont know,if you were to increase it you will get more power as the boost is raised but it also raises engine temp and if you do to much you can melt pistons and things like that.you also have to adjust the fueling to suit it as well i think.Im not that experienced in this field so i tend to leave things like this well alone until i have done some serious research.PLEASE dont touch it on my say so as i know nothing about td5s and you can really mess things up,if you have a look at a firm called twisted performance theres a whole host of things they do to get the most from your engine ok they are not cheap but when you weigh up what an engine rebuild and new turbo will cost you its money well spent.one thing i would do if i were to tinker with it would be to get yourself an accurate boost gauge plumbed in and make small adjustments but mark its original setting first,with that said i dont know how much boost you can safely gain and remain reliable im sure one of the big boys will let you know soon hope this helps mate sorry i cant give you anything concrete to work with best of luck!!