That was never entirely true, and I think is less so now. Hilarious example I noticed yesterday: Radiation Armor on Tankers has pretty meh baseline S/L resistance, even triple-slotted with IOs. It's better with Tough, but still only in the 70s. However, with both Tanker sets slotted and the proc active, it hits 90.00001 S/L resist.
I'm pretty sure that behavior isn't an accident here. But even on Live, Synapse and Castle mentioned being conscious of things like permahasten when balancing sets.
Being aware of things like perma-Hasten is one thing, but if we start balancing sets with IOs in mind, there can be only one result: nerfs, and tons of nerfs. For example, Permadom is incredibly powerful, there's no way it could be left alone. Softcap defense? Letting players hit in excess of 90% damage mitigation? That's where we were in issue 4, and we all know what that led to.