I am not even sure why you need to lower the priority. LAME will use all available CPU cycles even if it is running at the lowest priority. The critical word here is AVAILABLE. LAME does not slow down the windows interface, other programs, or anything else I run. I think it is because it is already at a low enough priority to operate unobtusively. Remember, if you are doing nothing else with your computer, then you WANT LAME to use all remaining CPU so it gets done as fast as possible.
As a matter of fact I have successfully ripped a CD, browsed this site, and run two external Lames in dos boxes at the same time. I have a pentium II running at 333MHz. If your machine is faster or newer, you could do even more.
I don't think you have a problem at all unless you see it cause other programs to slow down unacceptably. It looks like you can get everything you want by using the version 8 lame dll with the command line setting as discussed above.
Have fun!