For the grenade the wait time for it to detonate won't be effected by atk speed. It will just be thrown at the atk speed.
I think the concern with splash is that it can very easily spiral out of control. As it stands, the Reaver is absolutely necessary on certain towers because of how amazingly useful it is. With even more splash items, you wouldn't have to "pick" which tower to give splash damage to, you could just give it to them all.
And with the shotgun will take 8 seconds to reload after every 8 shots. And with the grenades take time like the pubs effect. It will take 4 seconds to throw it and 4 more to detonate but the recharge will take at least a minute to recharge.