I have a DIR-655 on my home computer and a DAP-1522 in my home theater. Want 300 kbs to the DAP-1522 (or as fast as possible) but I also have some older g stuff, laptops and a Squeezebox that don't support WPA. Can I get .11n AND reasonable security by running no encryption with MAC filtering on?
No.
No. You'd be far away from reasonable security.
No encryption is exactly that -- no encryption. Even if your network is locked down, I can monitor everything happening on it from the outside. It's like living in a glass house -- does it really matter how good or bad the locks are if I can see everything you do and say on the net?
MAC filtering -- MAC addresses are never encrypted. So if your security is based on MAC addresses, all I have to do is spoof one of your MAC addresses and I can get in. This "feature" is good for making your network difficult for you to manage, however!

Don't use it.
300N is also a pipe dream. Even if you do everything on the "stickys," the fact is that 2.4 GHz is too crowded nearly everywhere. Don't get me wrong -- this stuff is great technology, but it's been marketed beyond all reasonableness by all the players in the industry. You might get 300N with the DIR-655 if you live in a Faraday cage, but other than that...