A multiplier increase is, IMHO, better than a threat range increase. It's not just about average damage increase.
For reference, your damage increases by (size of threat range) * (multiplier-1) / 20. So, a 19-20/x2 longsword increases by (2*1/20)=10%. So does a 20/x3 weapon. Well, with Improved Critical and Keen it's easy to increase the first number, so anything that improves multiplier can make things nasty. Keen would add 10%, IC would add 10%, and then your Feat adds 30?
But as I said, it's not just about the average damage. I've got a Fighter who uses a Scimitar. It's Keen, he's got IC, so he threatens on a 12-20. When he does a Full Attack, he's practically guaranteed to get a crit (good thing he's got a Burst on that sword)
But, what happens if it takes a 15 to hit? His threat range effectively gets smaller, since you can't really do double damage on a miss. (Don't even think of saying that you automatically hit on anything in your threat range, or I'll Power Attack and Expertise for a billion and still hit on a 12 or higher)
A multiplier increase, though, will always do its extra damage. This is counteracted, in a way, by those crit-only weapon abilities that don't increase with multiplier (Vorpal, life stealing, the Feeder abilities from PsiHB). Even so it's still stronger IMHO.
Go with FireLance's suggestion: the multiplier only increases on the ORIGINAL threat range. If the unmodified crit stats added 10%, then Keen, Improved Critical, and your Feat will each add another 10%, just in different spots.