Greg,
As Clive mentioned, Don Goldman says his filters are good at fast ratios. I think the latest FAQ says they are OK at f/3 but lose 20% transmittance. At f/3.8 the loss would be small but if you're at a dark site a 5nm (or 6nm, Mikey!) would be nice for planetaries with NII.
Slawomir: I'm keeping the guide scope
Cheers,
Rick.