Ed,
While for many years most bullet companies calculated BC's, partly due to the very reason you suggested, many are switching to actually shooting them under controlled conditions. So it depends on the company.
On the other hand, recent research by Bryan Litz indicates the actual BC varies more to rifling twist than was previously believed. Actually, until fairly recently a lot of shooters felt too much twist "over-stabilized" a bullet, reducing BC, by Litz's research (involving actual shooting) indicates that over-stabilizing actually increases BC up to a certain point.
Litz's shooting essentially shows Hornady's listed BC for the 7mm 175 Spire Point Spire is spot-on, but I don't think he's tested the 150 TTSX. As noted earlier, however, I doubt its BC is the .450 Barnes lists. Among other things, Nosler's listed BC's are pretty good, and they claim .435 for the 150-grain Ballistic Tip, a noticeably sleeker bullet than the 150 TTSX. And Berger's BC (derived from Litz tests) for the .30 150 VLD is similar.