I agree with the Overall statement of this Post. Regarding a "GiveWell of X" type of organization I believe it would have to function quite differently, ideally only working on-demand instead of doing broadly aimed research for the following 2 reasons:

•  The target group of such an organization would have to be succeptible enough to cost-effectiveness arguments to be moved by their research, but resilient enough not to already pivot to a GiveWell charity instead. That sounds very nieche. Starting out with the mission of an established organization and convincing them that there is a better way to achieve their goal sounds like a more promising approach than "more broadly aimed research".
• In case this organization presents itself similar to GiveWell (easily browsable website with their main results, etc.) this could dillute the perceived effectiveness discrepancy to GiveWell charities. But then again most donors with the tiniest bit of alignment to EA probably Look at the numbers too much for this to become an issue.

I love this concept and I really hope it gets the momentum required to really take off!

One minor bug report: I was trying to sign up to GoodWallet in Firefox via my Google account, which caused the website to already fill out [first_name][second_name] as my suggested GoodWallet-pagename. However, this string contained more than 20 characters, which made it impossible to create this wallet. Trying to delete this string to manually choose a smaller pagename did not work, which got me stuck in this step of signing up. Retrying the same process in Chrome worked just fine. (Let me know if you need more details to recreate the bug/if there is a dedicated way to report these sorts of issues properly.)

As already suggested by many other users, I also believe that increasing the tax efficiency is crucial. I am no expert what the best way of achieving this would be. However, I feel like my own likelihood of using this tool would drastically increase with its (perceived) monetary efficiency.