I don't want to over-complicate anything for you. I likewise don't want to leave you with a less-than-total understanding of the situation here.
First, the answer to your question is essentially, no. Benzodiazepines will continue to exert effects in a basically linear fashion. I have never heard anything to suggest otherwise. A person can continue to take more Benzodiazepines and the higher dosages will continue to produce the same effects as lower dosages, but extrapolated.
The truth however, is that the issue is more complex. Benzodiazepines are not meant to be used by physically healthy individuals for periods longer than 4-6 weeks. The term "physically healthy" in my terminology in this thread means, people who don't have a medical condition like say, epilepsy. A patient with severe epilepsy might be prescribed chronic Benzodiazepines as the prescribers accept that a Benzodiazepine dependency is preferable to a life dealing with seizures. It's a cost/benefit decision. In short, Benzodiazepines are not a legitimate, chronic treatment for anxiety.
This statement can be seriously confusing for a lot of people, as most people without any knowledge of pharmacy would probably tell you that "Xanax is the standard treatment for anxiety". The reality is that most physicians will prescribe Benzodiazepines for periods of longer than the established 4-6 week period if they are willing to prescribe at all.
To come all the way around to your question, it's not strange for an individual like yourself to perceive their Benzodiazepine prescription as being less effective "pound for pound" as their dosage increases. Lower dosages are generally going to provide higher benefit to the individual. The desire to increase the dosage is normal and difficult for many to resist.
The common path is for people to increase their intake at an increasingly disproportionate cost/benefit ratio. The end stage of a serious Benzodiazepine dependency is basically a state of constant panic (Central Nervous System overdrive; fight or flight) between doses of Benzodiazepines with limited functionality in terms of their day to day life.
So there is no ceiling effect in true pharmacalogical terms, but if we're talking about the effects of these things on your real life, then yes, there is most definitely a ceiling effect and you're dancing around the line with your current dosage of Diazepam (Valium). I can't tell you exactly what to do, but I can tell you your best decision is to not escalate your dosage at all from here for any reason. Find a way to make this dosage work for you. Always know that dosage escalation will lead to nothing but more bullshit for you and your life.
