Slightly faster, maybe. Arty weapons don't reload fast though.
Ever seen artillery in action? Then, you would know, what i mean. Real artillery cannons fire about 1 shot/10 seconds. That's slightly faster than the GMHs reload, isn't it? [:D]
Shrapnel as opposed to the direct impact of a cruise missile?
Who was speaking of shrapnells? They would be completely useless against armor, i guess. I mean the sheer explosion blast, that would knock the enemy down.
10 to 25 damage to each titan, now that's overpowered.
That's why enemies would avoid to get two of their Titans into one hex at the same time. Also, for a weapon of that size and costs, it wouldn't be overpowered. In fact, it would be the most expensive and biggest weapon available on Titans (and therefore, the most destructive).