Since Warp is meant to be a drop-in replacement for GCC's cpp, do you plan to include a traditional (pre-standard) preprocessing mode? This has been a source of some agony in FreeBSD, preventing clang's cpp from fully replacing GCC cpp on some ports (mostly X11 related due to imake). So far the partial solution has been ucpp.
It seems that it's not so much about pre-1989 "code," but rather Makefiles. ANSI C preprocessors like yours will convert tabs to spaces, and Make doesn't like that. It seems reasonable to let this use case be handled the way it always has been: by using an old tool that still works.
The other major area of difference seems to be that pre-ANSI people used foo//bar to paste tokens (whereas now we use ##). If we're talking about C, that's easy to update; apparently some Haskell folks can't do that for their own reasons. Again it's a use case which is not preprocessing of C or C++, so it seems OK to ignore it if you're implementing a C preprocessor (as opposed to a generic macro expander usable with Makefiles and Haskell).
I wrote a 'make' program in the 80's (I still use it http://www.digitalmars.com/ctg/make.html) and it has a macro processor in it, but it is not like the old C preprocessors.
Warp does support the obsolete gcc-style varargs, but other obsolete practices it discards.
In any case, I haven't seen any Makefiles big enough to benefit from faster preprocessing.
http://gcc.gnu.org/onlinedocs/cpp/Traditional-Mode.html
https://wiki.freebsd.org/PortsAndClang/CppIssues
http://code.google.com/p/ucpp