VM_GROWSDOWN

The stacks for threads are not allocated with VM_GROWSDOWN [*], so you already pay the "price" for a fully reserved address space there. VM_GROWSDOWN apparently onlys affect the main thread, so the expenditure of additional 8MB of address space is really only once (and if the admin likes fine-tuning, s/he can always rlimit this further down).

[*] I don't see how GROWSDOWN would make sense for thread stacks, to provide any meaningful growth potential for them you would have to thoughtfully sprinkle them throughout the address space and carefully dance around these locations with other mappings.