With the mods mentioned previously.... i had no low end bog. Just the usual slight 4 stroke hesitation. As for the gear ratios.... I ran 13/52 on the 250 and had no problems with it. It suited me just fine. The bike came with a 54t rear from the dealer as he was playing around with it. I didn't like it in the woods. I found myself shifting and between gears way more than I liked. I went back to the 52t and was happy. I found that I could wind each (transmission) gear out further. I just rode the bike more like a 2 stroke and I was really happy. Maybe because it's been over a month since I sold/rode the 250 I am not remembering exactly correct, but I agree that the 400 needs the 52t rear sprocket. I just didn't feel I needed any lower than the 13/52 combination in the woods on the 250. Also, I very rarely saw sixth on the bike unless it was the grass track at Cooperland. Normally I would ride the 250 in 3rd to 5th here in the woods in Louisiana.
The bike really isn't slow. It's deceptive as it's faster than most peoples first impression. It can be a play/beginner bike, or you can wrap it up in the RPM range and really fly.
The one thing I did notice was that on the MX track, I did want the 54t for it.... but I very rarely venture out there so it was no big deal for me. Besides, I have the CR for than now anyway.