A 7m boat will cost more to run than a 5m one. The question is how much more and gas is only the start.
A bigger trailer - tandem rather than single axle, Brakes plus lots more rollers etc, at least twice the maintenance cost.
A decent tow wagon and the cost of servicing this.
Engine servicing - cost seems to increase in proportion to engine size for some reason.
More expensive fitout - 2 or 3 batteries instead of 1, windlass instead of armstrong etc, again probably twice the upkeep cost.
TAS - bigger boat needs more stuff to fill it, budget 3x what you currently spend.
In terms of gas you will spend more because you will go further, my previous boat, a 720 billfisher with 200 Optimax returned around 0.8nmpl at 25-26 knot flat sea cruise and about 1.1nmpl trolling so your 32nm trip to the noises would use approx 40l, say $85 plus oil not too different to your current setup.
The answer to the question is that the boat will cost a lot more to run and own but, as the owner of a largish trailer boat I recon its definitely worth it. It opens up infinitely more opportunities, places to go and is a logical step. Personally I'd go shared boat ownership rather than downsize now.