There are actually quite a few differences.
First off the 15 horse used a 16 cubic inch block, the 6/8/9.9 used a 12.6 cubic inch block, so you can "hop it up" a little but it will never make 15 horses.
Second, the carb is different - not just the jets, so you must replace the entire carb.
Third - the exhaust plate is different (larger bore on the exhaust tube) so the exhaust plate/tube (which is all one unit) must be replaced in order to allow the motor to "breathe out" properly or all the extra "intake" has nowhere to go and it won't develop the horses.
Fourth - the plate/tube that originally fit the pre 1994 models is no longer in production and while the new plate (94 1/2 and newer) will bolt right in its place, the water tube must also be replaced (the old style screwed into the plate, the new one presses in and can not be retrofit back and forth - so new water tube for the new plate).
Fifth - the new water tube has a flare about 2/3's of the way down the tube so it will not fit through the existing guide hole that exists in the exhaust housing. No big deal but you need an "installers" drill bit at least 24" long to widen the hole in the casting to allow the new tube to pass through.
So if you do all that you can turn your 9.9 into about a 12 horse = 300'ish bucks worth of parts = 150 bucks a horse and if you can tell the difference between a 10 (really) and 12 horse then you are far better than me (or the average guy for that matter).
(hmmm sounds like I have "up-horsed" a number of 9.9's for guys
)
Later model 9.9's were built on the 16 cube block and can be really converted to 15 horses, but anything from the mid 1990's back will never quite make it.
I have done quite a few "non-scientific" tests between 9.9's and 15's (I own both) and on my 14 foot aluminum, under varying conditions, the maximum performance (read top speed) difference I have noted is 3 mph (gps measured) and more often, closer to 2 mph.
Now that is kind of significant when your top speed might be 15 mph to start with, but unless you stare at the gps the difference between say 15 and even 18 mph is nothing you notice.
The earlier converted 9.9's (up horsed) often result in "no perceivable" change.
I always try to talk the requesting person out of the upgrade for all the reasons I list above and if they still insist I take their money and they lie to themselves about how much better/stronger/faster their motor is.
