I have no idea why Surly wants such a high bottom bracket on that bike. Maybe they are concerned about people pedaling through corners. If you're the type that pedals through corners, then maybe you should stick to Surly's recommendation.

All I can tell you is that I am currently riding a bike with a 279mm bottom bracket height in off road conditions. I designed and built that frame with that bottom bracket height. I sometimes ride that bike on singletrack trails with large rocks and log overs, but I'm a skilled rider who knows when to pedal and how to negotiate obstacles. If I were building a bike to use strictly as a mountain bike, I would choose a higher bottom bracket.

I've tried to be as helpful as possible, but I can't make your decision for you. You are the only one that knows your skill level and intended purpose for this bike.