I believe it's OT but is it really necessary to follow the recommended string tension? Just got a NS9000X and had it strung at 22lbs (main) 24lbs (cross) following the recommended tension for testing. Well, I'll say the first hits were a little weird (already got used to 25lbs) but after a while I got the hang of it. What I noticed is that I needed to exert more effort (more power) into a smash so the opponent won't be able to return it. Maybe it's just me or I still need getting used to it but according to my fiancee, she loves the racket because she's not feeling the vibrations on her shoulders when she hits hard. =)
Yonex "recommended" tensions are purely for warranty purposes - if the racket itself breaks while strung within this tension (provided you only hit the shuttle), it would be replaced. My advice is to ignore the Yonex recommendation (it's basically the same 22x24 for all their rackets) and string it to your preference. Professionals regularly go into the low 30lbs and rarely break racket frames - the strings, sadly, are a different matter...
well, on a totally different topic tensions ignored, I've tried the prince beast (your other racquet) and other prince racquets and it's definatly not as stiff as the ns9kx... I sport the mp100's and when I bought the ns9kx, it took me, and is still taking me quite the extra effort to get the power out. With stiff racquets, you can't be lazy and expect it to have the same power as the less stiff ones so it could just possibly the fact that the racquet is stiff and you need more muscle in the shots to generate power. As for string tension, I do my ns9kx to 30lbs but the higher you go, it becomes even harder to smash
Oh yes, I think this combination would be perfectly safe, especially if you use 2/3lb less on the mains (as you have here). A 900P carrying BG-85 is capable of power bordering on dangerous.