If you wanted to get more stack out of your aerobars, would you add more stem spacers or raise the pads through risers?
I realise adding ttem spacers would give you slightly ‘more’ than the advertised stack (depending on your stem angle, but not by alot.
Sort of depends on the bike and how good they did on the head tube shape. Probably risers.
Generally prefer raising the pads (“pedestaling”) over spacers.
Note that the approach you take will affect your reach too, not just the stack. (And 10mm of spacers != 10mm of risers for stack too…)
Some details: https://chris-lamb.co.uk/posts/effect-pedestaling-tt-bike-position
Raise the pads if the extensions move with them. Add spacers under the stem if they don’t.