What is it that you actually have a problem with, with this scanner? I'd trade my Canon 9950F in for your 646 at any time...
I have used both a 646 and 949 at a professional lab that hires these machines per hour and have always been impressed with the results (but I haven't yet done a drum scan...), way better than anything flatbed. I must admit I have never really tried to "tweak" the results, as again I was quite happy with results, both in terms of detail, color and sharpness (not oversharpened in my opinion, just good, no change needed in Photoshop). Actually, I'm not even sure if these machines used custom setting (except being well calibrated) or used the defaults of the Imacon software.
Yes, the 646 is waaaay slower than the 949 (about 3-4 times), but there is nothing you can do to improve the "productivity" of 646, unless you upgrade to the 949... It's probably a hardware limitation, maybe not so much the scanner itself, but the connection to the PC.
The best review I have ever read about an Imacon, is this one about the 949 by Giorgio Trucco:
Dead Link Removed