You could keep flagging to get rid of abuse and adjust the rate with the story upvote so that time on the main page is still a function of popularity.
On the other hand, low-scoring "show" submissions might especially stand a lot to gain from constructive feedback, something (as noted) they probably don't get when they don't make it to the main page.
Separating show submissions to a new tab allows you to use upvotes appropriately and distinctly. I would upvote startups as "news" in a different way than I would upvote startups as "review-worthy."
I see how spammers would create fake 'review my startup' posts to try and game the amount of front page time they got but if flagging is still applied to these posts then they would disappear quickly as people recognise that the submissions do not fit the 'review my startup' criteria and flag them.
Maybe I'm missing something obvious.
Perhaps an alternative might be reducing the amount of upvotes that a 'review my startup' post requires to get on the main page in the first place. Or perhaps some combination of the two.