Rates are pretty much exactly double unless you hit a max cap (which is usually only possible in TOA at 550+ raid level, but you're still at a 1/2 chance at that point)
I simulated TOA raids from level 300-550 assuming you get 30k points (max is 64k, so that's probably undershooting it), and the purple rolls are always doubled or higher.
ToB ones are easier, since it's a static rate in hm/regular, in hardmode for example your team needs to roll a value less than 40/100 to get a purple 1/2.5 for HM raid.
There was an issue where the boost was just not being applied to barrows though, but I believe that's working now as of the latest update
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