You should follow manufacturer's instructions and also follow your fire codes. I understand what you are saying about the visual space problem you'd encounter with a mantle two feet above the fire box. I believe there is a formula that you can use to determine according to fire codes how far away your mantle should be from the firebox it's based on projection of the mantle from the wall. For example, a mantle that projects 12" from the wall needs to be further from the firebox, probably 2 feet. A mantle with a projection of only 6" can be closer to the firebox. At one time I found this formula and determined that our 8" deep mantle could be 1' from the firebox and we were able to have more space above the mantle. Look up your local firecodes and see if you can find a similar formula for your area.

Update: I spoke with our inspector today. He looked up the regs and said that for anything combustible projecting more than a certain depth (something along the lines of less than an inch or so), we need a minimum of 12" clearance. Given that our mantel projection will be 8 or 9", I think we'll split the difference between the what the manual suggests and the bare minimum allowed in our county.