Yes, but is there a poll showing how any of the GOP leaders in the poll of their own party would fare against Hillary, Biden or Sanders?
Yes. RCP has the matchups. Clinton is very weak, and Carson, Trump & Bush are doing pretty well vs Clinton or Sanders. Biden would do very well if the US voted today. (Scroll down for various match-ups) http://www.realclearpolitics.com/epolls/2016/president/2016_presidential_race.html
http://www.realclearpolitics.com/epolls/2016/president/2016_presidential_race.html
Current General Election match ups.
Polls Clinton v gop candidates
http://www.realclearpolitics.com/epolls/2016/president/2016_presidential_race.html