The only thing that doesn't scale is your boiloff rate. If you boiloff 1 gallon in an hour for a 5 gallon batch, you will still boil off 1 gallon (or maybe a little more) for a 2.5 gallon batch in the same kettle.

In other words if you would normally start with 6 gallons preboil, then boil off 1 gallon to get to 5 gallons, you should start with 3.5 gallons preboil to get down to 2.5 gallons, not 3 gallons.

Hop utilization may be a bit better in this case as well, but it should be close enough where you wouldn't need to mess with it.