I'm really loving it so far! I can really see the potential here. Writing a level editor in only a handful of lines is amazing. I wish I had this about a month ago. =P
You said that HWA works but is a little slow. I have another HWA problem.
With HWA on, the levels draw in black & white. Collisions don't work as well, and my little guy falls into oblivion. Everything works great in Standard mode though.
As for level format compatibility, why not allow the extension to save maps as bitmaps, like the old Doom games? I believe Grand Theft Auto used this method too. I'm currently using some Surface objects to save map data as PNG images, which I then send to my artist (on a Mac) who can edit the maps in Photoshop too. It uses the RGB data to represent tiles.
Red is the tile, Green is the tileset, and Blue is any other data. Thats the format I'm using right now.
PNG offers some of the best file compatibility across many OS and programs. Of course a proprietary compressed file format is most ideal for distribution, but during edit time (when the file format may change a lot) its good to have a common-ground format that is easily convertible.
The Tile IDs and HWA compatibility would be ideal, but no pressure or anything. Otherwise its perfect. I'll be messing around with this extension in my free time to see what I can do with it as it is.