I think this is a lot like cars in that the highest top speeds are exhibited in 4th gear and not 5th. You are perhaps off of your power band. Rather than deal with the engine try experimenting with the front wheel. Use an aero one from a racing bicycle (just to see) and wear clothing that is close fitting.
I raced bicycles for years and I can tell you there is a reason for the tight clothing. For example, I used to ride up mt. rainier often and, on the way down, I would easily pass the cars, except on one occasion and that is when i forgot my cycling clothes and instead wore a t shirt and cotton sweat pants. I was so slow! The cars were passing me and there was no way I had the power to overcome the aerodynamic drag.
What you may find is that you will go well above what you had in the past.
Bicycle Power Calculator
look at the above site (found here on motorbicycling) to give you some idea