Based on the fact you have altered the drivers, you may have already changed the design intentions....if there ever were any. This would depend on the mfg'ers point of view.

Adding a port depends on whether you could live with them if it really changed the sound...as in horrible.

BTW, the 3 watt driver you replaced, may have in fact been suffcient, depending on the crossover design. Tweeters do not necessarily have to handle large power amounts...if my memory serves.