Hi Mike
The answer to the first question comes in two parts:
The 25 degree primary bevel for BU plane blades was something I evolved a goodly number of years ago now. It arose out of the challenge to more easily camber BU plane blades, especially those with high cutting angles, such as 38- and, particularly, 50 degrees. At that time, almost all users of LV bevel up planes would either turn to one of the 38- or 50 degree blades they sold. Achieving a camber on these, and especially the more extreme cambers used for a jack plane (such as a 8-9" radius), was a great deal of work. Cambers on BU planes need to be a little greater than on BD planes.
The idea was to only use a 25 degree primary bevel, and add the desired bevel angle via a micro secondary bevel (using a honing guide - needed as bevel angles on BU plane blades determine the cutting angle, and accuracy is more relevant now). What this did was to minimise the amount of steel needed to be removed to create a camber. It makes adding camber very easy to do. (Note that this does not invalidate the high angle blades sold by Lee Valley - they are appropriate for use is you prefer to use a straight blade. Some argue that they can achieve the same result by clipping the corners of the blades. I have never found this an effective method).
http://www.inthewoodshop.com/Woodwor...aneBlades.html
The second part of the question is whether there is a difference between a BU primary bevel at 25 degrees, and a BD primary bevel at 30 degrees. The answer to this is that (a) unless you are shooting end grain, where the bevel is 25 degrees, the rest of the time you will add a secondary bevel and the final angle will in fact be higher than 30 degrees for BU plane blades. And (b), the low vector of a BU plane blade creates less stress on the blade edge than that created on the higher BD blade edge. Tests I did on shooting boards, BU vs BD, revealed that the BU plane (with a 12 degree bed) far outlasted a BD plane (with a 45 degree bed).
http://www.inthewoodshop.com/ToolRev...tingPlane.html
To answer your second question: If you are using a honing guide, it does not matter whether you start with a hollow or flat grind. All that counts is the angle of the secondary bevel. In terms of strength of the edge, over the course of several years, I never experienced any chipping on a 25 degree hollow grind on a LV LA Jack used to shoot hard wood end grain.
Regards from Perth
Derek