I believe there are more differences than just the slide/barrel. SIG spent a lot of development time of the P229--they did not just slap a .40 S&W barrel and slide on a P228. I think considerably more thought and effort went into the development (frame and slide) of the P229 than you're allowing--in other words, it is not just an overbored 9x19 with a .40 S&W breech face like the Glock. Where, on the other hand, the G17 frame and G22 frame are completely interchangable--the only difference is the slide/barrel. I know it has not been plagued with problems the Glock has had (or that other "quick and dirty" conversion--the Beretta 96).
FWIW, SIGArms did give the P229 a new model designation when it was introduced (and it was introduced as a .40 S&W). They kept the P226 and P239 model designations when they introduced .40 S&W and 357 SIG versions of those models.