Why not both? Hire forrestv full-time to work on P2Pool and spin up another group (maybe headed by Luke-jr?) to improve GBT. Bitcointalk is making about $5k in profits per week (based on some back-of-my-hand estimates on undisclosed expenses such as the sysadmin that was hired along with the known expenses such as mods), which gives plenty of room to work with without even touching what has been stored up. Bitcointalk could currently sustain two full-time devs at $120k/year.
I don't want to eat into the reserves too much, especially with the forum software project unfinished. (The forum currently runs at a monthly deficit of ~90 BTC due to the software project.)
What else do you plan on using the reserves for after the new forum is done? If you plan to keep Slickage working for bitcointalk indefinitely, I'm totally fine with that as an answer to that question, since those guys do some pretty good work. Otherwise, it's kinda silly for a forum to have several million dollars in reserve.