If you are looking for expansion, then I would choose another film. Bergger BPF and HP5 are not noted for having a lot of expansion potential. Films that have greater expansion potential are FP4+, Efke, Tmax 400, and JandC Classic 200.
You didn't mention what format that you are using...but it you go getting too rambunctious with the dilution then you start getting into grain problems. I have found grain apparent with BPF200 4X5 at an 8X10 enlargement when I used Pyrocat at 2-2-100 dilution to try to gain expansion just as you are.