The over design does not come from manufacturability solely. It is related to all kind of uncertainty, such as device modeling, environment modeling, simulator accuracy, specification creep (i think loos-electron coined this term), etc., etc..

I think the least costly method would be to add digital circuits to provide some tuning afterwards. Though it adds more work on verification...

I agree, there are lots of uncertainties and you should always add a bit of 'sandbagging'... and yes, trim can help.

I know what you're saying, loose-electron. Sell the really good parts for more. If 95% are within spec limits then great. However is that enough? If all parts over process, temp etc etc are within those limits is that good enough? The product engineer is always banging on about getting good Cp and Cpk numbers, especially for chips designed for an automotive environment. So is it enough to be within spec, or must you design to half spec limits in order to meet the Cp & Cpk numbers.. I guess Cp and Cpk are used because the process can shift and you don't want it to sit outside your spec limits... but if we get a Cpk of less than 1.6 or so, I think the parts are binned - I must find out..