I finally made it to level 40. I had thought that the rate that I earned XP at would slow down at some point after I hit level 32, but it's still flowing just about as fast as ever.
I've found that if I just hit groups that contain nothing but +3 level minions the rate that I gain xp at stays steady regardless of my level. The bigger the group of +3 minions the better.
Playing for 3 or 4 hours a day makes it possible for me to level about every other day if I do nothing but solo. If I group with lower levels to help them out, or run missions with the SG, the rate drops off dramatically, but friends are more important than xp.
UPDATED:
-Sarcophaguy. Level 40 Mutation Energy / Energy Blaster. Pool powers are Hasten, Super Speed, Combat Jump, Super Jump, Stealth, Health, Stamina, and Swift.
My next power selection will be Grant Invisibility followed by Phase Shift.