Yeah, the sound a little on the stiff side as is, but the next weaker spine is a 500 and that would definitely be underspined at 29", or at least right on the upper limit. The 400, being the next stiffer should be just fine.

If your in a quest for more speed there are other arrows with a 400 spine that are lighter in the mass weight department. You just have to look around for them. One I shoot is Gold Tip's Ultralite series. I shoota 500 at 27", with a 27" draw, and about 60# is as hard as I can push them with the hard cams on most of today's bows.