Your original post describes the core as "warmish" after a WSPR session ... maybe some clarification of what temperature "warmish" is would be helpful. A core that runs 'warm' should not be cause for great concern ... most core materials can tolerate even moderate heating just fine. 'Hot', on the other hand, would of course be another matter. As pointed out in an earlier post, some of the heating may be from losses in the copper windings.
One disadvantage to stacking cores (keeping the same number of turns) is that the winding inductance doubles from the original design. This will likely increase the leakage inductance ... one negative effect being an increase in switching transient ringing.
Jay W1VD WD2XNS WE2XGR/2
p.s. I'm looking into getting more driver boards made ...