Good to see that straight pool is helping you out. Stick with it! I don't know if you have skilled straight pool players in your area, but seek them out if you can. One of the benefits of straight pool is that you are more likely to get a game with a very good player for cheap or even for free. People who play straight pool just love it that much! If you don't have the opportunity to watch and play better players, get your hands on some Accu-Stats tapes of straight pool matches. The play and the commentary will provide you with a wealth of knowledge about the game. You may also want to get Jim Rempe's tape "How to Run a 100 Balls". In terms of books, check out Ray Martin's 99 Critical Shots in Pool, Arthur "Babe" Cranfield's Straight Pool Bible, Phill Capelle's Play Your Best Straight Pool, and George Fel's Mastering Pool.
As I was rereading your original post, I was moved to point this out: What you are calling a ceiling can be better visualized as a plateau. In our pool development, we are riding a bicycle up a mountain that consists of a series of slopes and plateaus. Think of a plateau as the place where, depending on the quantity and quality of work you put in, you can gather speed and momentum to better climb the slope, pedal slowly and stay on the plateau for a long time, or coast and stay on the plateau forever.
During your plateau stages, you are improving various aspects of your game in what we may call quantitative development. That gradual and imperceptible process of learning and absorbtion, during which you may appear to be at a standstill or even going backwards, eventually culminates to cause a significant and perceptible shift, or qualitative development, in the level of your game. It's like boiling a pot of water--for a long time it would appear that nothing is happening, but...... The rest is obvious.
You'll have more plateaus and slopes to travel in your journey, and I wish you the best.
Peace,
VIProfessor