why would you weld plate to the subrame? A: is permanant, and B: It has the possibility of warping while welding. Pressable Bushings seems SOOO much easier. And choosing Alum over steel is a no brainer; It has soo many characteristics that are better: Corrosion, easier to machine, you can press it easier using the freezer method, and the cost of Alum is cheap! especially when you factor in tooling. you will go thru so many more inserts cutting steel then alum. ill use 1 insert to do all 10 bushings. start factoring time it takes to make steel ones, its longer; ie more money on your part.
If you know how to weld, warping is a moot issue. Welding is no more permanent than pressing an aluminum piece into a steel sleeve and then left to corrode. If you want to remove the plates, you cut the welds and remove the plates. Easy as that. Yes...Aluminum does corrode. That white powdery stuff which you see on the surface of old aluminum parts is oxidation, and you get a ton of it when dissimilar metals are pressed together. Machining would be minimal with steel discs. You wouldn't even need to machine if you're good with a cutting torch or plasma cutter and have a drill press (or a hand drill, for that matter). Aluminum is much more expensive pound for pound than steel...been to the metal yard recently? The price is easily double, and to make bushings from solid stock, you'll be removing over half of your purchased aluminum in the form of lathe turnings.
As far as welds cracking, I guess this goes back to knowing how to weld. Some of the welds I've seen on this site would certainly scare me in ANY application. The load on the bushings isn't very condusive to cracking anyway. Most of the load is transferred through compressive force (tightened chassis mounting bolts) directly into the unibody. Any sheer forces would be spread evenly between the 6 mounting points, and 12 plates (1 on top, one on bottom).
The subframe breaking as a result of being hard mounted? I suppose that point loads could be increased, but it certainly wouldn't be any more of an issue between a welded plate type mount, and a pressed bushing type mount.
SG