stephmp_bonusroundtime 15 (15 is the max) would probably be the most you could do, just config-wise.
If you want to push it as far as using server mods, it should be possible to raise the max as long as FCVAR_REPLICATED doesn't fuck with that, which I doubt it does.
I know there's a generic SourceMod plugin that lets you change ConVar flags (mainly used to disable the sv_cheats requirement) but I'm not sure if there's one that lets you fuck with default/min/max, I'm gonna start looking, otherwise it should be fairly easy to implement.
What I'm getting at is, if this is wanted, it can be achieved. Most likely.
EDIT: sm_cvar was the command I was thinking about, according to docs it doesn't respect min/max bounds, I'll try this locally.
EDIT 2: ^ didn't work, this does; https://github.com/ldesgoui/unbounded_mp_bonusroundtime/releases