It does sound like they are putting it up to the 2.5si specs. I never really understood why they didn’t put it up there in the first place as the engines are almost the same, the e89 is bigger, heavier and it’s not like the 28i is only a few horses up.
I have a 2.5si, my dad has the 23i sdrive. I have driven it once for about half an hour and I did notice the extra power in mine, but it’s hard to say how much of that was the feel of the cars in terms of size weight etc.
I’d probably go for it depending on how well regarded the tuning company is. I’d certainly be doing my research for the best tuner and would travel a long way for it if If I had to.