@tigga when you say "no ramparts keeping you out" do you mean a base that doesn't have the exits zoned out? If so, I had also thought that boosted swarm attacks were more effective against bunker bases but now the turret damage seems so high. IRRC @bonzaiferroni had used boosted swarms on bunker bases before.
If the scenario that I described above does lead to "2 creeps vs 6 towers is often a losing battle", then 2x healers + an attacker also loses and it would take 3x of these healers + an attacker to withstand full turret fire. And of course the healers have to sustain adjacent positions, and their TTL have to stay in synch long enough.
I've seen @o4kapuk use siege squads of three with a full healer in between and his leader and tail gunner have 10 boosted tier 3 heal parts each. But they can't be pre-healing all of the time in order to perform other actions like dismantle or attack. Frankly, I'm a little disappointed that tier 3 boosted attacks like these can be so easily countered with random focus fire as active creep defense should be more or less required for the counter. Maybe I just need to watch more sieges but I'm surprised at how any siegers out there are taking bunker rooms down, especially if they dig into bunker bases, especially with active creep defense