A partial answer. I recently bought a Baer with the factory two-piece magwell. Don't know who makes the magwells for Baer, but I assume they're made by one of the mainstream aftermarket manufacturers. I then bought a set of aftermarket grips from Esmeralda (www.esmeralda.com) that were made for use with magwells. If you go to her website, you'll find that the grips are seperated into "with" and "without" magwell. I assume all of the quality grip makers list their products this way. She also has an option of with or without ambi safety cut-out.
I was concerned that the grips (kinda expensive) wouldn't fit, but was pleasantly surprised. The fit was better than the factory grips with no "fitting" requires. On the other hand, I bought a set of inexpensive grips to use on a pistol that I had installed a magwell/msh on. The fit was awful and required the removal of a lot of material from the botto, of the grip.
Based on my limited experience, its a toss-up as to whether aftermarket grips are going to fit out-of-the-box, and you're not going to know until you have the parts in-hand.