for cold / fire sorc.. What effect on immunes in hell? Is it worth it?

infy is always good for elemental damage char
but, lol - pvm fire sorc sucks, blizz (cold) isnt so good, too
i luv lit sorc and i promote it to u

anyway infy>

so it wont give me the same boost as for a light sorc?

Infinity isn't enough -res to break most cold/fire immunities. It is, however, enough to break most lightning immunities.
That's why it's used on lightning sorcs most often.

Infinity, by far, is the best choice. Huge fire or cold or (fire and cold) boost. Very few monsters have both fire/cold immunity.
Also, Infinity allows you to stat only 1 point (or none with Tal orb) into Cold mastery : 19 or 20 more points into fire tree !

From experience, good fire/cold with Infinity is better than light.

this all day long

light/cold can be very strong too
either way; dual sorc > single sorc

infinity