A popular alternative to the Delta screamer on the SK-6 is the retail packaged Thermaltake 6025. They claim it as rated as 31 CFM 27 dB(A). There's annecdotal evidence that these ratings (unlike the ones for their Volcano fan) are accurate as far as CFM goes.
I've only seen it at handful of vendors though.
Say Gizzo, I found lots of useful info at your site; I like it a lot. One question though, where are the high CFM 60MM Deltas?