Yeah I believe you are correct. Only so much can be done.
I didn't think of this before but, if I replace the R5 2.2M with a 2.7M, it brightens it up just slightly. I tried a 3.3M and it was definitely brighter, but lost too much "punch".
So I'm going with a 2.7M at R5 and a 47pf at C3. As well...