Yes, use 5.5mm bullets. I just recently switched to 6.5mm Castle bullets, and damn, they are nearly impossible to get off sometimes, they have so much holding resistance. They do seem to be loosening up a tad though, so I curious as to how they will hold with use and time. They do stay stone cold, noticeably over Deans (at least old ones), the Deans that got replaced felt slightly warm after a run.
But who knows, Deans might still be the ultimate connector, because who knows how long bullets last compared to Deans; and for most applications Deans really do the job great. I can see 2 years from now 12s lipo setups being the new '6s setup', so currents at those voltage levels will never exceed 50 amps or so, and Deans are just easier to connect and disconnect, and they could provide more than enough current handling at point.
EC3 and similar connectors are good too.
As well as these, Hyperion's new bullet connectors, they look great:
http://aircraft-world.com/shopexd.asp?id=5960