Our daughter took a couple of exams with them last year, before the November 11+. Both times she scored around the midway mark & we were told she was likely to be borderline. I agree with Koala's assessment in that it is helpful to give them an experience of taking exams and to highlight areas of weakness, although our daughter's biggest weakness was concentration and that is not so easy to fix!
In the end we were delighted when our daughter gained a place at King Edward Handsworth - and not with the lowest possible score. I don't think the Shirley centre takes age into consideration when they mark and as our daughter has an August birthday, this probably worked in her favour. I shall be using their mock exams again as my Yr4 son and Yr2 son approach the 11+.
I agree mocks are useful for identifying problem areas.
My daughter is hoping to go to Handsworth too, can I ask if you used the named centre for mocks only, (with tuition elsewhere), or did you also use that centre's tuition (computer and in person) with the mocks?