The "Super" Sniper supports boost. It has a MAP sensor built-in and I'm sure there are other additional features, but a quick scan of the Holley website will tell the story.
The 1250 is the boosted power it will support. The difference between the 650 and 1250 is the 1250 has 8 injectors where the 650 only has 4.
Yes, you can run the 1250 model at much lower power. Think about it like this, even a 1250 HP engine makes very little power just cruising down the road. The system will adjust automatically.
69 Camaro
400 SBC, ProCharger D1SC