This is a common theory, but if you look at the math of the rocket equation, a very large percentage is for pure horizontal velocity, with a small percentage of gravity losses for getting from the surface to the orbital altitude, an even smaller percentage are losses due to aerodynamic drag.

I haven't done the math, but in general, the benefit of something like stratolaunch is much less than you'd expect

Here's a great (but complicated) reddit post on this topic:

https://www.reddit.com/r/KerbalAcademy/comments/63nzq6/how_d...