I think the guide guns are neat. I think they are really missing the mark by not making the magazine tube full-length. It would make sense to increase magazine capacity in a back-up type gun. I would not hesitate to pick one up, try it out. If for any reason you don't like it you won't have trouble selling it.
Yes, muzzle blast and kick have been over-rated I'm sure. The .45-70 is not a "plinking" gun for most, especially when loaded up. It kicks hard in any rifle, but not so hard that you can't shoot it well, and get it sighted in real good. A ported rifle is louder, but yes it's much worse under a tin roof.
Having said that, I'll stick with my standard length Marlin .45-70, which is "pre-safety" and has the straight grip, both big plusses for me personally. It is VERY accurate.
Sean, I was shooting the 400 grain Speer at that same velocity as you, until I shot a deer at close range (25 yards) that jumped up out of the cat-tails...that bullet blew up really bad. So, if you use that load for hunting you might want to reconsider, I don't think it works very well for hot loads. Ideal for normal 45-70 velocity I think.
I have since changed to the 350 grain Hornady RN which I shoot at 1700fps.