Why, yes Wilbur, there can be too much water in the mash tun. But can there be too little as well? Let's start by listing the ways wort can get into the boil kettle. In no significant order:
1. Batch sparging - mashing at water too grain ratios of ~1.25 per pound and then adding water in large batches to rinse the wort into the boil kettle - faster than fly sparging - no arm or pump needed.
2. Fly Sparging - a spinning arm held above the wort delivers water in an even composition so as to gently wash the sugars from the mash - slowest and most efficient.
3. No Sparge - just that - you simply add the full volume of water to the mash tun and drain into the boil kettle.
4.Brew In A Bag - much the same as No Sparge except that the mash tun and the kettle are the same vessel. Rinsing the grains lowers gravity.
The BIAB method is as efficient as any of the above methods given proper consideration for equipment relative to the size of the mash and consequent gravity of the finished beer. However, high gravity beers will begin to show a drop in efficiency due to the amount of sugars held by the grain relative to the water to grain ratio - essentially, the thicker the mash, the lower the efficiency. So as more grain is added to the kettle, which puts the wtg ratio below the 2 quarts per pound ratio, efficiency will begin to diminish.
There is sufficient evidence to prove that attenuation is unchanged no matter the volume of water as a factor in "normal" range gravity, but efficiency is another story. Thinner mashes transfer more sugars into the boil kettle than thicker mashes.
This link is for the geeks out there and fully substantiates that higher water to grist ratios produce a more efficient extraction. Thus - BIAB saves time, space, money----and is at the top of the efficiency curve with the rest of the lautering methodologies!