I've been working on a homebrew Brikwars RPG based on the some of the mmos created by Spacetime Studios (Star Legends and Pocket Legends, specifically). Since you guys seem to be having some trouble with the XP system, I'll share what I've developed and confuse the issue some more.....uh, I mean help out.
I considered creating a CP-to-XP system, but ran into the same "kill one tank and instantly level up" issue. So I decided to award XP based on how much damage the target could deal instead. Infantry are only worth 1 or 2 points, while heavier vehicles are worth more, but not exponentially more. I'm still fine-tuning the formula, but it's roughly 1 xp for each d6 of damage.
Of course, since my system is built around infantry scale combat, it might not take well to being scaled up to larger battles. Shaving off a portion of XP earned by comparing the PC's damage compared to their target might be a workaround. Honestly, I can't really think of any situation where the players could get themselves into a situation where they'll be knock out tanks left and right, without having an army alongside to sap some of the XP (and they do manage to single handedly crush an tank platoon, they probably deserve the XP).
"There's suspension of disbelief and then there's insulting my fucking intelligence"