This barrier exists, but the "highness" of it depends of the quality of the weed. With schwag/mids (what i used to smoke) is very easy to reach a point where you smoke, and smoke, and dont become more stoned. With good quality weed, its harder to reach the barrier, but its still there, and as your tolerance increases, it becomes easier to reach it.

Or, if were to say exactly what happens. If you smoke twice the amount you smoke usually, you dont become twice stoned. Like... you smoke 2x and became 1.5x stoned. If you smoke 3x, you become 1.8x stoned. If you smoke 4x, you become 2.2x stoned... and so... the "barrier" effect comes with large amounts of weed.
Like... if you smoke 10x more weed, you become 3x more stoned. If you smoke 20x more weed, you become 3.2x more stoned... If you smoke 30x more weed, you become 3.3x more stoned... so, strictly speaking, there is not a real "barrier", but what happens is you need to smoke much more to get a small increase of the level of the high.

Note that this numbers are in no way exact. They are only an example of what i mean.