One big drawback to an E21 brake booster is a noticible reduction in power assistance. Some people use a 325IX brake boosters but a 320I booster is cheaper and will allow you to use your stock master cylinder. Sometimes in hot weather the engine will ping even on a 91 gas so if 92 or higher octane gas is not available in your state you probably shouldn’t consider this option. One downside to this high compression is you have to use the best gas money can buy.
Upgrading injectors will probably make even more power because the engine leans out and stops making power after 6200 rpm. With M3 cams (ether 3.0L or 3.2L) the engine dynos at around 240-250 crankshaft horsepower with stock injectors. The combination will give you a compression ratio of around 11.25:1 according to my estimates. One trick of making it high compression is reusing the 2.5L pistons (only if in great condition with new rings) with 2.8L rods and 2.8L crank. There are plenty of 2.8L core engines out there for as little as $200. One good way of building a low budget high performance 2.8L is purchasing a 2.5L M50 engine out of a 525I (so you can get your oil pan) and stroking it. You might want to consider an option of building a high compression 2.8L engine that will perform on par with a 3.0L and probably cost less.
The only problem with 95 S50 motors is that they are becoming old and it is very hard to find one in a good low mileage condition for a reasonable price. The simplest and probably most economical engine to swap in is a 3.0L S50 engine out of a 95 M3 primarily because it already comes with most of the OBD I hardware you will need for the swap. In my opinion the smallest engine you should put in your E30 is a 2.8L M52. If your main goal is making your E30 fast, do not do a 2.5L swap. First I would like to give a word of advise to anyone doing this swap.