I think "lean C!" is not really what one wants to hear if asking for a Basic dialect.
I program C/C++ and Java at work, and Amiblitz in my free time. This says something, doesnt it ;-)
On 68K, C is not faster than Amiblitz since you can always use very easy inline-asm for the ciritical parts. Also, the Amiblitz compiler generates fairly efficient code that does not fall behind GCC. But Amiblitz is not a BASIC dialect.
The biggest concern is anyway not the syntax of a programming language. Given enough training, you can get used to almost anything. It is the runtime that makes the difference. Amiblitz is not faster (to develop) or easier than C per se, if you need to implement everything via AmigaOS calls. It gets faster if you start using the runtime.
For C, you have, apart of the spartanic clib, a lot of "runtimes" in form of linker libs, like SDL. The problem here is, that they dont know each other, and you have to do a lot of dirty work to get them together. SDL is quite a complete runtime, so this is less of an issue here. But SDL can also be used in Amiblitz.