Sticking grenade on mines, make the explosion more effective?
Yes but as you add more grenades on a mine .... the more the speed of spider decreases but the damage increases.
Rachel wrote this mutator which I've seen on a server - we ended up riding the things.
http://www.angelmapper.com/gamedev/ut2004/infospidersteroids.htm
It would have been even better if the amount of damage required to kill the things increased with size.