The A/C leeches power in two ways....first it is more load on the engine drive/accessory belt. Second, when run it creates more underhood heat which raises engine temps (still in the safe zone mind you) but costs power just the same. Exact losses can differ alot but a 10-15HP power loss is not at all out of the question. You will not hurt anything by running it to redline with the A/C on. Just think of it as sacrificing a few HP/lb-ft for the sake of a comfortable cabin in the summer months.

Hope this helps,
Lee

PS for max acceleration hit the "ec" button as mentioned above. Turn the full a/c back on when it suits you.