In an aircraft like a Cirrus with a fuel injected engine, there is much more that one can get from a high power check. To do this you need a gauge that reports EGT values for each cylinder. The test is preferably done with the engine mixture leaned (this accentuates any faults). On switching to a single mag at say 1700 rpm all EGTs should rise and eventually become steady. If one of the EGT's drops this usually signifies a plug fault. If all EGT's drop then it is likely to be a mag fault.
It is even better to conduct this test during flight!
