Would Sivir be OP if she was manaless but...

No.

I'm a fan of building manamune on Sivir, actually.

infinite waveclear, slightly OP

Sivir just needs a better ult and she'd really be top tier. I'm not saying it's not useful, but she needs something help her deal damage in trades, not a tool to run away with or catch runners.

Doubtful she would be inherently "OP" if she no longer had mana costs

but it would be take away a little from her gameplay

since her w would still have the same cooldown, yeah, she'd actually be totally broken.

In a word: yes.