Hey Otto,
There is absolutly NO way that changing your valve adjustment will affect your cruise RPM. The engine may be louder due to more clearance in the valve train leading you to think its turning faster, but it will be turning the same rpm in the same gear at the same speed that it always was.

If the engine really is turning faster, it is due to some completly unrelated problem, and with a manual trans the only place slippage is possible is the clutch.

Cheers, Chris