No, it is designed to last 2x as long. So it does 2x the damage by lasting 2x as long. If you increase the DPR, it is doing more than 2x the damage - thus it should be worth more than 2x the XP, but it is not.Why do you find it against the spirit of the elite monster?
If it has to be two monsters into one, and has the same actions, reactions and bonus actions, one would need to double the damage. However doing that would certainly result in a monster with too high damage output for the given CR rating, but a +50% boost should be a significant but still manageable increase
Now, I am 100% with upping damage to compensate for less than a 2x increase in HP