Earlier in this thread, I promised to report back with comparative info wrt 585V7 vs ancient 3Com...
First - as Kitz predicted, static IP address wasn't a problem, I just allowed the network to allocate and, lo and behold, it allocated my familar 'static' address.
Second, I'd been patiently waiting for 15 days of continuous sync before making trying the 585, on the hope my target SNR would drop in that time. Sadly, we had a power cut just before day 14 elapsed and I'm still on 15dB. Grrrr. I'll never know if another 36 hours or so would have made the difference.
Finally - thanks to the previous paragraph (a 'silver lining'?, a more meaningful comparison was possible, with the same target SNRs. Immediately before powering off the old 3Com, I resynched it at 1884. Ten mins later, I powered on the 585 which synced at 2464 - that's a worthwhile and convincing improvement, I reckon.
So far, the 585 has reported NO CRC errors - the 3Com was reporting about 150-300 per day. It's only been an hour so I can't really draw any conclusions from that, but I'm keeping my fingers crossed that the 585's going to be more stable as well as faster.