I have seen it mentioned that programs running on PI1 B/B+ will run just fine on the PI2. I have several PI1 machines and one of the issues is that compiling software can be very slow (cross-compilation is too advanced for me). If I buy a PI2 and install raspbian on it, then compiling C++ programs will be several times faster, but will those binaries run under raspbian on the PI1 machines?
depends on the compiler flags. If you tell it to compile for the older core, then in theory there should be no problems. Note, however, the main thing that slows down compilers is I?O, not CPU speed. So you may be better off simply getting a faster SD card, or using a fast USB disk drive for your compilations: the latter option will also avoid excessive wear of your SD cards!