[plt-scheme] Bit manipulation in PLT Scheme

Hi all,

I'm having the need to work in a somewhat low-level with bits and
bytes, which I've never done in Scheme. I'm curious about if there's
any performance or memory penalty by using bitfields in Scheme
(SRFI-60) (is there any alternative to this in PLT-Scheme?). Will a
bitfield of 64 bits occupy 64bits in memory or will I have a penalty?
Will a logical 'and' between two bitfields have any performance
penalty when compared to the same operation in C?

Is there anyone experienced out there working with bits/logical
operation that wish to comment on their experience, performance, etc?

Thanks in advance,
--
Paulo Jorge Matos - pocm at sat inesc-id pt
Web: http://sat.inesc-id.pt/~pocm
Computer and Software Engineering
INESC-ID - SAT Group