Cold lights always need to be oversize relative to film format. For instance, I use a 14X14 V54 head
for 8x10 film. A good diffuser will help, but is not a substitute for the ideal grid size. I've noticed that many cold lights marketed for a particular model of enlarger per mechanical fit are not in fact ideal with respect to light distribution or eveness. The second problem is whether your enlarger lens itself is giving even distribution (doubtful). You should be able to compensate for both problems simply with edge and corner burn-in; but if you can't, you might need to look at the diffusion system itself and how to improve that.