Besides, if you're looking for a high-performance Scheme that fully utilizes all available system resources, there are definitely better options available. :)
As you probably want to combine these valuse together into some structures representing the various language constructs, an additional byte to represent the type of the structure, and thus the type of its elements, would also be needed. Than you could do away with the extra bits representing the type.
I just think this is premature optimization and making things unneccessary complex especially for your readers who might want to learn something from it.
As I said further down this thread though, it's a 'speed' thing, not memory storage thing, the common belief is that boxing is slow, because it was slow in java, but in reality boxing is a) a mostly acceptable trade-off that only loses out in extreme cache-limited situations, and b) something that could be optimised away anyway.
I agree that a lot of boxing can be optimized away, but often it also can't.