I said "I haven't seen any comparative polling of the possible contenders for May replacements except within the Tory Party itself".
On looking around, there doesn't seem to have been any polling done at all about whether anyone but May would perform better in a match-up against Corbyn.
I did find this:
ICM also asked about possible alternative leaders to Theresa May, underlining one of the problems the Conservatives have – in every named case (Johnson, Rudd, Hammond, Rees-Mogg, Patel and Green) people thought they would do worse than Theresa May would at the general election. The only person who the public thought would do better than May was a generic “someone quite young and able who is not currently in government”… which, of course, is a recipe for respondents to imagine an ideal candidate who may very well not exist, especially not among the select group of people with a reasonable chance of winning the leadership of the Conservative party.
http://ukpollingreport.co.uk/blog/archives/9948
For context of the poll mentioned in the OP, here's a roundup of polls since June of May v. Corbyn:
https://en.wikipedia.org/wiki/Opinion_polling_for_the_next_United_Kingdom_general_election#May_vs_Corbyn
May's generally been ahead, but they're tied in the latest YouGov/The Times poll.
The same sort of roundup for Tory v. Labour:
https://en.wikipedia.org/wiki/Opinion_polling_for_the_next_United_Kingdom_general_election#2017
It's trending to more or less neck and neck still - +/-3% isn't safely outside the margin of error for most polls. In terms of a real GE result, given the way our electoral college usually works, Labour would need to be significantly ahead percentage-wise in order to stand a chance of winning a majority.
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