Are they consistent?
Yes, they are but not all of them. The ones that say "quickstart" are being upgraded to be more consistent ball that lasts. I have tried them and I think they are the same. They fuss up quick and turn orange like rather quickly.
Do they last?
Again not all of them. I have found the Dunlop stage 1 lasts the most and is most consistent. Because they are pressurized in a can. Wilson does have the green dot which is second best but not as good as the dunlop, IMO. Prince has loose green dots in a bag but they do not bounce as high as the dunlop, nor feel as solid. Prince also has the green dot in a can but the ones I have seen are not pressurized, that might change.
Are they better than the old dead balls in my hopper?
They are better, IMO , because the old balls in your hopper are heavier and are all over the place in terms of bounce and felt/feel.
Can I just use a bingo marker or some stencil ink on an old dead ball?
Yes you can, which is a nice idea, you can separate the different types of balls, in terms of life. I have many sets of balls that I use for different occasions.
I have two large supermarket baskets for my HS tennis team, which have all types of balls you can imagine.
I have one 75 hopper that is new so the HS kids can practice on before matches, once they are used on several practices they are thrown in the large supermarket baskets.
The used match balls are also thrown into the two supermarket baskets, etc..
For my two kids, one is 6 and the other 9.
The 6 year old has a hopper of very old flat yellow balls and a hopper of red balls and a hopper of orange.
The 9 year old had a hopper of dunlop green dot ball and a hopper of Orange practice balls which are just as heavy as yellow only they bounce lower, NOW we have a hopper of yellow balls that have been used once.
For my wife and I, we have a hopper for the ball machine that is pressureless treton Micro X, they last the longest out of all the balls IMO but they are so expensive that I am now on Wilson Team pressureless balls because they are 1/4 of the price.
Hope this helps, i know I am a ball junkie
, I have over 40 students, two kids, one plays competitively and a wife who plays nationally.