Ive never heard Rainbow so I cant speak of that coaxial, but I have heard the REVs and theyre great coaxials!! Not too many better ones out there, and they go for a good price!!
The coaxial though that I really like and the one that Ive listened to that sounded the best was the Focal 165cv1, they go for 180-200, im sure there is a few that can compete with or could be a bit better, but these speakers sounded great, sounded almost like a component set with smooth sound, good midbass, and crisp cystal clear sound, and the best part was when I first heard them they were running off the HU and sounded really really good!!
But for a rear fill the REVs should do the job and will sound very nice, you really dont need to spend close to 200 on Focal coaxials for a rear fill