@oristarA,
Ori, 'on par' means equal to, even with. It's a golf term. Each hole on the course has a par of either 3,4 or 5, add them up to get the course score or par. If you play they course or the whole game 'on par' then you have evened the playing field.
The sentence is fine as is.
Try this..
'despite dire warnings from the U.S. Treasury on Thursday that a U.S. debt default would lead to a financial crisis
even with, if not worse, than the 2008 financial crisis.'
'despite dire warnings from the U.S. Treasury on Thursday that a U.S. debt default would lead to a financial crisis
equal to, if not worse, than the 2008 financial crisis.'
The word/verb be is unnecessary.