If you find that you do need 550mm blades check out:
www.airborne-models.com
They have Fiberglass 550mm blades in semi and symmetrical for $28.00. I have been flying thier 600mm blades for a while and they are good bang for the buck, about the same price as woodies without worring about warpage and the plastic covering coming off. Thay have a carbon fiber reinforced leading edge with some wood spars inside, the aft portion of the blade is hollow. I have had the chance to check out what the insides look like on a couple sets of 600mm blades from them . Thay are white gelcoated also. They are not as rigid as the more expensive composite blades but they cost about 1/3-1/4 . The balance has been very close on mine as I use the koll to check all mine.
Vince D