Bath would certainly be worse off without our acquisitions - but I'm not sure how much.
Hill is probably first choice, but the way Read has stepped up, it's far from nailed-on (would Read have stepped up without Hill around?). For this season, of course, Hill's been injured for almost all of it, so you can't put much of this season's performance on him.
Barbeary personally is a huge positive for Bath, and so is Coetzee. With those 2, I suspect the biggest positive from each of them is that ability to allow R&R and keep them both fit and motivated.
Lawrence is absolutely first choice OC, but without him, we'd still have 3 perfectly good centre options in Redpath, Ojomoh and Butt, and we'd probably have brought someone else in. It'd definitely be a step down, and certainly impact our ability to rotate. I'd also add that I think Lawrence is a big part in the resurgence of Cokanasiga - they both occupy so much of a defender's attention, that you really have to put 3 defenders on the 2 of them, because at least one of them is going to be made to look silly (and then there's a 1/3 chance that one of the Bath pair will be made to look silly as well).
For Hill and Barbeary, there's not really a step-down to the alternative. For Lawrence there is, but there's also a step down in experience, with Mojo and Butt are improving game on game.