I guess I would start with the belief that ALL of the products you have used so far are excellent against German roaches. Excellent, that is, if the cockroach and the active ingredient manage to come together in the same place and for a long enough period of time. Gentrol, of course, does not directly kill roaches, but affects their population over a longer period of time by stopping production of new roaches. Avert baits work only if the roach is interested enough in this new food resource to feed on it, and if the restaurant has the sanitation problem you suggest, then the roaches already are perfectly happy with the food materials they are currently eating. You probably will get "some" of the roaches eating this bait or any others, but most of them may ignore it. The Arilon should be a perfectly good contact insecticide, and hopefully you are applying it directly into crevices - not along baseboards or exposed surfaces. Since the roach spends 80% of its time tucked into crevices, holes, voids, and under things that do not get moved, those are exactly the places you want your active ingredient to be too. The roach has to spend enough time directly in contact with the a.i. to be able to absorb sufficient a.i. to kill it, and a quick walk across a "band" of the material applied to walls is not likely to do it. 

SANITATION! Is there a reason this customer is not cooperating in that area? Have you and he actually discussed the need for sanitation with respect to pest management, not to mention just serving healthy food that is not contaminated with bacteria that grow on filth. Toss a cockroach problem in there too and you have a potential Petri dish of pathogens. The roaches crawl into floor drains, toilets, on rodent feces, and into and onto a lot of other surfaces where some really nasty stuff grows..........and then they walk onto the counters where our food is prepared. Just not a good idea. 

I once heard a manufacturer's rep talk about problems like this, and his final comment was "you might just want to drop them as a client". What?? You drop THEM because they don't show an interest in doing their part in the whole process of eliminating filthy roaches? Sure. If their expectation is that you are 100% responsible for eliminating these roaches, but don't plan to remove the conditions that brought this problem in the first place and now are supporting the roaches, then you will continue to be frustrated. And, if some intolerant restaurant patron finds a roach in her noodle soup then she may look around and see who she can sue over it. 

Perhaps on your next visit, rather than again spraying insecticides onto dirty surfaces, take a written Sanitation Inspection Report and do a thorough inspection of this facility. Note everything that you see that is a "contributing condition" that helps the roaches to do well here, and then sit down and discuss it with the restaurant manager. If he is receptive then work out an action plan of who will do what and when it should be completed, and on your next visit see if any progress is being made. 

In addition, YOU have many sanitation and bio-remediation products that you could apply to dirty surfaces, to drains, to grease traps, to fill in crevices, etc., and offer this service to the customer as well. This is just as much a part of the overall roach management as is the application of insecticides. And, if you have NOT been applying your residual materials directly into the crevices and voids then that would be a better place for them.