When i bought my car the oil was above the high limit by a fair bit. The level was right up to the twist in the dipstick.
Is this bad ? Can it cause a loss in performance ?

Its not a good idea to leave it as its likely to blow out oil seals and gaskets.Also it extreme cases cause the engine to smoke and yes perform badly.

Yes, it's REALLY bad. :o

Drop the oil and fill to correct level with a top quality oil without delay.

Mike.