You are correct. The biggest gains of a slightly slower burning powder being given 'time' to work in a longer barrel mean the actual shot is smoother, recoil is not quite as sharp, and the bullet accelerates a little more slowly, yet still achieves max. velocity. Side bonus; barrel throats tend to last longer.

For the 223, the ideal practical length is 20". If absolute accuracy is the goal, then 22" is a slightly better option, and 24" barrel length would be most ideal for the scenario you asked about. With really quick burning powders, you might see a stagnation of barrel pressure development as the bullet is just leaving. NO; the powder will not all burn, and the bullet slow down. It happens way too fast for that. The perfect scenario is for the primer to get struck, the powder to start burning, the pressure build and throw the bullet down the barrel, and pressure would build to a peak pressure of about 55,000 ish.... as the bullet reaches a point of maybe 2" from the end of the barrel. Pressure ideally would be very consistent, from shot to shot, and yes, the pressure at the very end would be slightly higher, if the charge is built correctly. A powder which burns too slow would leave energy on the table, as it were. Bigger flame report from the barrel, and the charge behind the bullet may be still building lots of pressure, which can adversely affect the tail end of the bullet, and degrade accuracy.