terpjr, just thinking out loud about your setup. This is all pretty much theory since nothing has been said about actual amount of heat to be transferred (other than 3000 sq.ft) your actual circulators and flow rates on each side of your FPHE, etc. Since you are using baseboards, close approach temperatures are o.k. and by upping the amount of plates you end up with lower pumping resistance, enabling your circulator to move more GPM on the boiler side. Again, no mention of the actual flow rates of your individual zones but lets assume you might need 50,000 btu/hr and your zones get 5 gal/min. You very well could be suppling 15 gpm on the boiler side which would help keep load side temp. high.
To my way of thinking this would work fine but there is a reason the manufacturers rate the units as they do. Without a certain amount of turbulence the self cleaning properties of a FPHE disappear so more chance for build up. A 70 plate might get you a closer approach temperature(you might get more gpm out of your circulator)but at greater expense and shorter life. If you have any trouble heating your house, maybe a higher head circulator would work. I just looked at a 50 plate that was rated at 500,000 btu/hr, and a 70 plate that was rated at 825,000 btu/hr. (definitely overkill since the mfg. claims best efficiency at 84.5 gal/min)
Another thought is that none of this would affect your wood consumption only your delta "T's".
Hoping to learn something here so will stay tuned.