My Kimbers have always been flat. I'm used to it that way. I think arched would maybe be better for very large hands. I know Wilson makes one that is slightly angled. I guess the idea is that it is supposed to make your hand ride higher on the grip safety. That type of mainspring housing makes some sense to me provided you have an extended beaver tail.
I find the arched to be a bit more comfortable, but not enough to get excited about. I also shoot equally poorly with either. I think the reason so many pistols come with flat housings these days is that the stock Gov't Models came with arched so it was an easy custom touch.
When I purchased my Brolin Arms 1911 Government L45 it came with a flat housing and yes, I shot low. So I "upgraded" to an arched housing and loved it. Then I added a Chip McCormick High Ride Beavertail safety with "memory bump." After shooting my RIA 1911CS with the flat MSH and finding I shot well with it, I changed the MSH on the Brolin full-sized pistol back to the original flat MSH and found that it now made no difference. The high-ride beavertail and memory bump seem to have made the arched MSH redundant and unnecessary.