Having it on Sunday hasn't been a problem before, nor is it now. You don't fix something that isn't broken.
I think the bigger thing is just, "Can PPVs get bigger numbers on Saturday" type of question. If it isn't broken, don't fix it. But you should always be looking for ways to improve. Look at TV Shows, for first 1-2 seasons, a show might be on a certain day/time and do great, but then network moves them to diff day/time and they do even better.
Wouldn't mind WWE trying out 1-2 PPVs on Saturday and seeing how those numbers do. There's more people in WWE Universe overseas(meaning basically everywhere that's not north or south america), so WWE should be having PPVs that could be drawing to those places alot too. If PPV on Saturday Night leads to like(just making up numbers), 30,000 less people in USA ordering, but like 50,000 more outside of USA ordering, then I'd consider that move a success.
I think in todays world, Sunday night has alot more tv shows getting big numbers than ever before(sunday night football,sunday night baseball,along with new episodes of TV shows on all the big 4 USA network). Saturday Night isn't known for being tv night, but WWE PPVs get like 200,000 people buying anyways, so saturday night won't hurt it that much, and like I said, it'd be helping out buyrates overseas more than anything.
So you can't just look at a WWE PPV for being on Sunday or Saturday night, you also have to look at it as being sunday or monday morning(or afternoon) in many other places in the world. TV Shows is one thing(meaning to care about USA ratings over international ratings),but buyrates really doesn't matter where they come from. A buyer in USA is giving the same(I believe the same?) amount of money as a buyer in England or something. Money is money, no matter where it comes from.