If v8 is fast enough for anything that I might want to do with it, further performance improvements (as opposed to other changes) are not going to be really high on my wish list. That's why the performance case doesn't work for me.

Other people might have different tradeoffs, of course. Which is fine. But nothing I've seen the Dart crew say yet explains in plain English who those people might be, and why the particular set of tradeoffs in Dart is better for them than either v8 or, say, some other language reduced to strongly typed bytecode for PNaCl.

They haven't reached them, that much is obvious since Lua is every bit as dynamic as Javascript and LuaJIT is orders of magnitude faster than V8.

The other browser vendors have built fast JavaScript engines as well, and they seem to think that Dart is unnecessary.