Being the author of Cibyl, I might be biased here. Anyway, I've looked at the java bytecode generated by the axiomatic C compiler, and it is not efficient. NestedVM and Cibyl both works by compiling MIPS binaries and then translating the binary into Java bytecode. It's surprisingly efficient, with the main problem being memory access of 8- and 16-byte values (which needs to be done in multiple steps).
NestedVM and Cibyl have slightly different performance characteristics, with Cibyl typically being faster for integer-heavy workloads whereas NestedVM handles floats and doubles better. This is because Cibyl uses GCC soft-float support (though using "real" Java bytecode floating point instructions) while NestedVM translates MIPS FPU instructions.
Cibyl is also more targeted to J2ME environments, although it's definately useable on other platforms as well. My guess is that you would have more luck with either of them than with the Axiomatic C compiler.