With just the 2 x 25" pontoons, I don't know if the 115 would be enough for you. I have the express package, and I'm pretty marginal with the 115, although much happier now with that new Spitfire prop. Without the center toon, you might just be throwing good money after bad. Like Carl mentioned, you should talk to your dealer ASAP about trading in your boat and see what they can do for you. They want you to be happy, and with a 2013 with not many hours on it, who knows. I know you probably don't want to hear that, but in my opinion you NEED to know this in order to make your decision.
Another option is to trade in for a slightly older boat that has 3 toons and a 115 or 150. Again, mention this to your dealer. You never know.
But, gut tells me you will be paying a lot for the 115 upgrade and you still won't be happy with the performance on just the two toons. The 150 is not an option for you with your current setup, so you are stuck.
You could also post your boat for sale on Craigslist. You don't have to sell it, but you might learn what it's worth (which is only what someone will pay). Listing it will keep your options open as you learn more and decide what to do.
Good luck. Tough one.