Also, I understand the potential to not recoup the investments needed to produce Linux versions. To mitigate this, have you thought about running a Kickstarter? There are projects that have raised millions of dollars on Kickstarter. While the recession is perhaps not the best time to run a Kickstarter, there definitely is hunger from Linux users to have a full-featured and user-friendly photo editor on the platform.
The goal could be to make Affinity Photo run well in Wine. A native Linux port (released as a Flatpak, so that you don't have to worry about dependencies on various distributions) could be a stretch goal. Other apps could be further stretch goals or separate Kickstarters if this first Affinity Photo one turns out to be successful.
The benefit to running a Kickstarter would be that there wouldn't be much to lose. If the campaign goal isn't reached, then you as well as Linux users like me will at least know that there isn't much of a market for Affinity on Linux. If the goal is reached, you will have a guaranteed payout that can cover the costs of porting as well as give you a healthy profit (depending on how you set the goal). And then any later Linux licenses that you sell separately will just be pure profit.