If you read St. John's Street Cycle website, they will explain in details that 26" wheels aren't slower than 700c or 27".

However, "efficient" 26" wheels means wheels that accept relatively narrow high pressure tires (35-37 mm, or 1 3/8" - 1 1/2" width, at 90 psi), so depending on the model of mountain bike you have, you may need to get new rims.

Your speed is also governed by the amount of air resistance, so you should ideally look for drop bars... which means new brakes and probably new shifters because your current shifters and brakes are incompatible with drop bars.

All in all, unless you really want a convertible bike, say, to bring only one bike on holidays, you should probably forget the idea.

Regards,