Man, I can see how your position could be frustrating. It usually takes me about a month an a half to go from low season average speed (12-13mph) to high season 17-18mph averages and I am close to 200 pounds and been riding for one year. 4 years is a very long time. I did notice a 1mph average increase in speed when I went from my Felt z85 to my current Carbon Roubaix but that said I think it was probably a mental thing where I was pushing like hell out of excitement. So my 2 cents is that you will most likely pick up an extra 1mph but that wont get you in a comfortable 15mph zone. Also, fit is textremely important and now that I think about it, that could have been the reason for the 1mph increase. The roubaix fits me worlds better than the Z did.

Good luck, with all your hard work you deserve a new bike