Surely the big problem with this (and the wider site concept, much as I generally favour it) is that the donating users who say they also want GIMP to be as good as Photoshop don't all have the same idea about what constitutes better UX. So you get a high proportion of dropouts from the funding subscription, as well as disagreements with GIMP's existing user base and team about whether they forget their original roadmap in favour of merging your patch, assuming you can still afford to finish it.
(Key difference with Kickstarter etc is that people are pledging for a specific solution rather that for people to solve a problem, and the cash is paid up front rather than a promised monthly commitment. And still there are issues with execution of many projects, particularly once third parties with different goals are involved. "I also need GIMP to be as Photoshop" on the other hand, is basically asking for a "nice unicorn" and hoping both that unicorns can exist and that everybody else involved - especially the unicorn-breeder - shares your definition of "nice" ones)
(Key difference with Kickstarter etc is that people are pledging for a specific solution rather that for people to solve a problem, and the cash is paid up front rather than a promised monthly commitment. And still there are issues with execution of many projects, particularly once third parties with different goals are involved. "I also need GIMP to be as Photoshop" on the other hand, is basically asking for a "nice unicorn" and hoping both that unicorns can exist and that everybody else involved - especially the unicorn-breeder - shares your definition of "nice" ones)