Just did 6, 1.5lb roasts with my Necchi convection top(Galloping Gourmet CO with Necchi 5700rpm adjustable sewing machine motor on the fan). I had just been using the split GG top(single fan speed w/variac on element) while I was learning how to drive

With the Necchi I boosted convection(adjustable, also with a high and low range switch) at about 290-300F and was able to keep a good 20+degrees/min rise with a much lower ET by 10-20degress. 460f just before first, 465F MET. Merely adjusted convection during and after first to control advance rate. Fortunately adjusting convection didn't change the ET as the circulation is contained.
Ed