This is the only info I have on the use of propylene oxide.
"PROPYLENE is used as an ignition accelerator particularly with nitro (up to 20% by volume with pure nitro) where noticeable increases in power are possible. Also promotes easier starting and smoother running when blended with methanol (up to 5%)."

Judging from that, the amount you've been using wouldn't have any noticeable affect. You could try using 3-5% acetone which seems to be quite helpful especially in zero nitro fuels.