I don't have any direct knowledge for use of the centered e36 m powerflex bushings on the xi. Many people swear by them for the XI. I think they were likely the best option before Powerflex apparently redesigned the xi bushing and corrected some of their earlier issues with this bushing.
Maybe there are other differences but from what I can tell (I'm sure I will be corrected in a very polite manner) The e36 m and the xi versions appear to be very similar in design and material, A firm rubber outer bushing with a Poly blue/purple inner bushing. I don't know if there are differences in durometers in the materials. Maybe someone who has used both can chime in here but i suspect they are the same. The main difference touted is that the e36 m version is centered where as the xi is offset slightly as per the OE FCAB. To some this is an important difference, maybe especially if you track your car and/or have otherwise altered the geometry. There is much debate on this point. For me I choose to trust the BMW engineers and I use the xi version which keeps the OE geometry with my stock OE ride height. My car is a daily driver and it aligned beautifully and handles really nice I'm very pleased.
The other difference is that the new XI bushing design fills the entire gap between the lollypop carrier and the shoulder on the end of the arm, where as the E36 M version leave a gap of about 10 mm when installed. For some this is not a problem, but some have resorted to filling the gap with a hose clamp to keep the bushing in place.
I may be missing some other important distinctions and maybe others can fill in the gaps. I just raise these points of distinction between the e36 M and the XI specific bushing so that you and others can make an informed decision and decide if this is a proper modification to your car. Despite the differences in the two designs I think most everyone who has use the powerflex bushing has reported favorably, though I am sure some will disagree