Firstly, I think you're right choosing a Mira product as I think they have to be pretty much the best on the market and the Excel has to be the most popular in their range, and I sell and fit more of those than any other. Surprisingly enough though I have no experience of the BSM.
Of course as you say, the normal BSM's give poor performance in low pressure systems because of the plumbing arrangements. However in a high pressure system these problems don't occur so I think you are making an informed choice. Performance wise I wouldn't expect anything less than the normal Excel model. My only reservation is with the big blob sitting at the end of the bath.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!