1. The frequency, depending on whether it's a highpass or lowpass will run for sure. The general rough guide is that if you use a 8-ohm driver in a passive crossover made for 4-ohm drivers would be that the highpass would be double, and the lowpass would be halved. The crossover slope WILL remain as it is(6dB will remain as 6dB, 12dB as 12dB and so on), as there's no change in the amount of components. As for using 2-ohm drivers in 4-ohm passives, it'll be half for highpass, and double for lowpass. Like i said, general guide.
2. There WILL be a gap as the frequencies doesn't match anymore, BUT, if it's a 6dB, the gap won't usually be that big due to the shallow cut-off.
3. There might be a mismatch of efficiency between drivers due to different tweaks in certain passives whereby it's MADE for the driver packaged together, things like Zobel, Notch Filter and efficiency correction takes into account for these.
But after all these are being said, sometimes, it doesn't necessarily means it can't be used, a driver rated as 8-ohm or 4-ohm doesn't necessarily meants it's DCR is the same as rated, neither does it mean it stays at that impedance thru-out music, best bet for seeing and testing whether the passive can work with your drivers is to test, especially with music you are familiar with, who knows, it might just sound right? :P >:D
