When the clutch starts slipping the load is removed from the engine, this would be like you going for a full boost run in 4th / 5th gear and pressing the clutch in, the engine needs to be working under load to generate the exhaust gas flow to drive the turbo, the harder you work the engine the more exhaust gas you get the better your boost/turbo responce is, that is why in 3rd,4th and 5th gear you will see much better boost responce than in 1st and 2nd, when the clutch slips the load is removed from the engine and the exhaust gas volume is lowered reducing turbo output. Out of interest is your clutch slipping in the higher gears only? (3rd, 4th + 5th), the only reason I ask this is because a while back I had simmilar symptoms on one of my scoobs but in any gear, I found a shaft had popped out of the rear diff and it felt like the clutch was slipping and the engine was very unhappy, popped the shaft back in and all is well now.
Cheers Iain