Hiya mikeymike
ok, there were a few questions there so I have split each paragraph up into each question :-)
1/ I am not entirely sure what you are after to be honest with the CPU usage side of things - are you trying to get more free CPU time back for the windows side when you switch inbetween? If so, then there is a option within the 'Misc' tab in the WinUAE settings that allows you to 'pause emulation' or 'run at a lower priority' (what I use) when you switch back and forth between the two OS's
2/ I am unsure about this one tbh, I have never seen that happen (I have left the emulation P96 settings at default)
3/ I have never actually tried using a modem connected to the PC as a modem for the emulated amiga. What I do is just select the 'bsdsocket emulation' option within the Misc settings tab option screen again, This has allowed me to go online and do emailing, irc etc without a problem. Doing it that way means that you dont actually need Miami or any other TCP/IP stack installed.
Dont know if that was of any help,
Sam Thomas