re: high cpu utilization ... there are a couple other areas that you could check but we're talking a lengthy, drawn out back-n-forth here, so I'd recommend opening a case with Sybase tech support if you're still seeing those high cpu utilization rates with such small cache hit rates (~200K/sec).
I wouldn't be surprised if one of their recommendations is to make a sizable reduction in the number of engines (besides, your Sybase sales rep has already made his/her commission on those 64 licenses! ouch!)
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If you have an ongoing need to use disk mirroring then by all means leave it enabled.
But if you don't need it then turn it off the next time you get a chance to bounce the dataserver.