Yes there is a penalty for manual spoilers. Don't remember what is.

On a somewhat unrelated note, I don't trust our dispatchers to figure out our landing performance.

A couple years ago, I found myself in a similar situation. We were supposed to go to an airport with an 8,000' runway. The only way in was an ILS, to minimums, at night, with a 10 knot tailwind. Ordinarily, not to bit of a deal, except for the braking action was poor due to snow and ice. My very "helpful" dispatcher had calculated we only needed 7,500' to land and stop, which included a 15% safety margin factored in to the number for which I don't really remember why. According to the tables in the book and QRH, under our anticipated landing weight, braking conditions, and the tailwind, I came up with closer to 11,000'.

We didn't go. The dispatcher got really unhappy, called me all kinds of names, and told me he was going to call management and tell them I refused to fly. I told him to go ahead, and I still wasn't going.

We eventually got in, several hours later, after they had been busy plowing the runway. I never heard a word about it from anyone in management.

Our dispatchers will try to force you in to anything. Safety here? It's just a meaningless word.