Either way I don't see you getting it working 100% fine due to the default behaviour of the boiler being suspect. Have asked a few questions on the forum for people who say they have it working 100% fine but never get a full answer to my questions. Quite possibly with a commercial circulator it may be possible for CH but DHW generation I just cannot see how it cannot suffer from micro firing when nearing full tank regeneration
Well, I have a 415 which works 100% and doesn't micro fire. The important thing is to match the flow rate with the first minute's firing (about 75% of max boiler output) and subsequently d0. However, you're stuffed from the start if your boiler is significantly over sized.
A 28Kw boiler requires a minimum flow rate of 20 l/m at 20c but you can get away with about 15 l/m if d0 is set <= 75% of max output and d2 allows for sufficient cool down.
The pump only needs to match the index circuit and therefore there isn't necessarily any benefit in using a beefier pump, only disadvantages (high water velocity, noise, higher operating costs). In fact, I'm using a 15-50 on the minimum setting.