Is 5.5bb/100ev over 25000 in nl5z enough to justify taking shots to nl10z? Should I continue in nl5z?

+13 buyins is enough to indicate you're doing something correct, but not enough to signify much of anything.

No because 25K hands is not a sample.

but feeling comfortable in skill when playing 5NL and having the bankroll to play 10NL is enough to justify taking shots at 10NL.

When should hero take shots to next level asuming he has the bankroll?

if you see some players you know well from your own stakes observe the table

bonus points if they are not good players, then sit instead of just observing

that's the most surefire way

another is to find one table with biggest wait list, if you can understand why that table has such a big waitlist then add yourself to the list, if not, keep observing - if there are a lot of people wanting action at the table and you can or can't see why then that's a decent indication

but really, it's about bankroll more than anything, someone correct me if this is no longer the case, but the gaps in skill between adjacent stakes isn't that large and only between other sharps

you'll still have bad players at all levels, hence why people who can afford it move on up