A check here with a few huge projects shows the loading time for one of them is down from about 2:40 to 1:30 minutes. This compares a really big project with 32bit vs. 64bit Xojo IDE. Probably due to faster code generated by the LLVM compiler for 64-bit and faster string handling functions.
For the plugins, I expect we have updated plugins ready before 2017r3 ships later this year, so everything should work smooth. I can’t predict if older plugins will work, but I expect to modify my plugins if needed.
The blog post doesn’t say it, but Xojo may drop 32-bit IDE and only allow you to compile 32-bit apps on a 64-bit computer. I have no idea how many Xojo developers still need 32-bit IDE, e.g. running on a Windows 10 in 32-bit. Using the remote debuggers you can still debug on 32-bit, but may no longer run the IDE on it. And you can of course debug a 32-bit app on a 64-bit computer, but no longer debug on a 32-bit computer without remote debugging.
* you can get on the beta program by signing up in your account settings when you have a Pro license.