overall the sensitivity of a 1x15 and a 210 is the same.
to increase the sensitivity of your system and reduce distortion you need to add more speakers. Matching speakers.
unmatched speakers have a different phase response at different frequencies so they will cancel and comb out, given peaks or dips in the overall response. And instead of adding to the overall system volume or sensitivity of the system. It will stay the same and include combing and phase cancellations. Making the extra money and time spent a waste
Just save your money and time and buy another matching 1x15
this will improve the overhead or your speaker system, and the extra tone and presence of 2 drivers will be a big improvement.