Actually... that's not quite true.
First, you can't do that without shaders.
Second, the way GameMaker currently works is SO bad, that this WILL give a major speedup.
Third, while it's true that object batching will help you draw many more
models at once, what I said is that you'll probably KEEP the object count to what you have now, but be able to UP the number of polys in a model. currently, you create a mesh, and then submit that mesh with a texture. This (by definition) is batching large meshes with a single render call. GameMaker doesn't currently do that. In fact... GM's model drawing is SO bad, this should be a major speed boost. I'd be really surprised if you can't up the geometry in a mesh by a factor of 10-100.
Depending on what you draw, and HOW you draw, this will provide plenty of power. Not having multi-texturing is a bugger, but if you can submit large parts of a world in a single call, then you have far more chance of keeping up your frame rate when you have to draw it several times.
I've done nice terrains before using a single (large) texture using around 320K polys, where it runs quite happily at hundreds of FPS. This would be much better than the current 1-2K polys you currently have. On top of this, the speed you get back from having to continually build the model, you can spend doing other things - possibly even drawing more models....who knows.
Also, having done quite a few tests on "instancing" not long ago, I can tell you that unless you pre-can a whole load of data, CPUs are now fast enough to submit almost as many models. This isn't to say GPU batching isn't better, just that you really REALLY have to know what your doing in order to get the speed up, and folk using GameMaker (on the whole) won't. So this should be just fine.
This won't be a magic bullet, but it should be a big boost for "large" models.