"The big issue changing flow IMO is the bottom of the lifter cup. I guess hole does have some but it depends more on the radius of the bump om the bottom of the lifter cup.."
True. The black cup is also used on the Caddy lifters but those have different [more expensive] internals. The customer specifies how much oil & the lifter is designed accordingly. Pushrod cup dimensions can also be specified so that's why some are deeper than others for example.
So while they might all look the same they're not. I sell the Delphi lifters that flow the same as the "B" lifter but don't have the black cup so they don't look the same.
The Hylift liters use the groove cut in the bottom to meter the oil.
How much oil is enough? you don't really need a lot to for the rockers but if you're also trying to cool your valve springs you'd want a little more. In a HD application having too much oil to the top end would cause the lifters to tick at idle so they find the happy spot that keeps them quiet & keeps the rider happy. Lifters typically need 30 psi to operate effectively so if you only have 10 psi at idle & they start ticking you know why. It bothers some people more than others. I'm hard of hearing so it doesn't bother me at all

 

Just wanted to update my findings on Comp lifters.. I went to the Comp cams Pro Magnum short travel 875-4 lifters and have been running them for about 7,000 miles without issue and they easily handle 6500 rpm.. Even with roller rockers the top end is pretty quiet..

Edited:https://www.summitracing.com/parts/CCA-875-4#overview