Persians already have an elephant speed boost UT but I thought about that too.
You think you could afford all the upgrades, the UT in castle age, and have enough eco to produce a bunch of elephants all in castle age? I doubt anyone would ever get to that point very often.
Thats my point. If you do that, it wont even be worth it at that point. This would weaken the Khmer ele's biggest strength; its's speed. It would also make other civs elephants in castle age a bit more viable, and in imp too.