So, I will put my $0.02 in. I am also predominately a lefty, so I feel your pain.
I was looking for a 150mm petty a few months back. I posted a query like you. After lurking on these boards for a number of months, I kinda formulated an appreciation for a few frequent contributors who had opinions and habits similar to mine, and had a lot more experience with the breadth of knives on CKTG and other retail sites. Two such contributors told me to get Fujiwara Nashiji 150mm Petty: http://www.chefknivestogo.com/fujiwara4.html
. It was $50 beyond my target range. I compromised on another petty with tons of character and really nice fit and finish, and it was $10 or so above my target price.
After holding one of the Nashiji knives from a friend (albeit not a petty, but I gotta believe that the fit and finish and 'character' of the blade translates from one type to another), I feel like a fool for not spending the extra cash. Moreover, compared to the pricing for the 240mm gyuto, the 150mm petty is a bargain.
As for grinds, most pettys I have seen are more or less 50/50. If you ask Mark at CKTG, he can double check for you.
Last, that UX10 you have - did you have it sharpened over to 70/30, left hand bias? I believe they come out of the box 70/30, right hand bias. Just curious. I have frequently used a UX10 gyuto that had been re-ground to 50/50, and had no problems with it steering.