You can change the valve guide SEALS without removing the head. The valve guides themselves are part of a head overhaul and require the head to be removed. Once they're installed, clearance has to be checked to the valve stems and the seats re-ground.

As for the reduced power, you could be having issues with the turbo failing to spool properly or fuel supply issues. It's worth checking how much boost you're making, you should be making 12-14PSI when the engine is under heavy load such as accelerating or pulling up a hill. When's the last time you changed the fuel filters? Replaced the rubber fuel hoses? Air in the fuel supply will certainly reduce power.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]