Originally Posted by Rattlebag
Fat tyres will also help with any brake reach problems
No, they won't. The width of the tires doesn't bring the rim any closer to the fork crown.
And the 4mm BB drop is really pretty damn negligible. So is the drop going to narrower tires (another 2mm? Maybe?). As long as you aren't trying to carve a super-wide slalom course with 175mm cranks you should be fine.