As it is now, a good player with a grenade can pretty much pop the OM at will (not on all maps though!!), all the tubes in the world wont melt him in time, and tubes actually do a fair amount of damage.
Im not suggesting any increase in damage or area of effect, but I would really like to see the slowing effect upped a bit, maybe slow as much as 40-50% ?
I know a lone marauder is also capable of ripping up a human base, but a marauder need time to do this, the grenadier only need to run to his target and press middle mouse button, and turrets seem to be a LOT more damaging than tubes, so 2 turrets near structures can foil any lone marauder, 3 tubes near OM cant do anything to stop the lone grenadier.
So .... if the lone grenadier was slowed as he entered the base, he would require more time to reach the delicate target, leaving the tubes more time to do their thing if he chooses to ignore all defenses and run straight for OM, and allowing the defenders to get more than 1 hit in before the grenade is dropped.
Increasing the slow effect should have very very little effect on a REAL human attack, where more players attack the base together, since these attackers usually use their grenade to pop the defenses around doors and such.