@gutreactions: In practice, the per-item tip would bring down tips for the waitstaff at expensive restaurants, but I think that is an acceptable outcome. The physical demands of the job, viz. balancing plates, walking to and from the kitchen, walking to and from different tables, are all essentially the same no matter how fancy the restaurant is. The difference is in the mental faculties. How much expertise a waiter demonstrates regarding menu items, viz. ingredients, preparation, sourcing; how well they can remember orders; and how friendly they are toward patrons are all mental differentiators which are difficult to quantify. That's why I suggest adding additional tip based on a percentage of the per-item tip. This allows patrons to adequately address those special qualities.

@Meat guy: As for the significant tip reduction on expensive bills, this seems to me like a real, practical problem, but not one that is the responsibility of the patron. A waiter at Per Se who thinks they deserve a much larger tip than one at a local Applebee's, despite their performing no greater a task, is simply spoiled by the current unfair system. Granted, the reality is likely that the Per Se waiter will perform their duties with greater diligence, but that is where the percentage service addition plays its role. At Per Se, you might decide that their quality of service is worth 50% of the base per-item, or even more. The key is that you don't tip them more simply for the expensive meals you ordered, which they had little to no contribution or input into.

Lastly, I totally agree that there's nothing barring leaving a $10 tip on a $20 bill, but social pressures do not demand such behavior, which means that this rarely ever happens. Much more likely is that regardless of service, patrons leave ~$4 on a $20 bill. While we can say that there's always the option of leaving a exceptional tips, I want to focus on the general social norms of tipping and their general practical impact on dining experience.