Yep, good idea changing both since this is always recommended. When you change the tappets you will need to run the engine in again so don't go over 3500rpm for at least 500mls. Colombo e Bariani make a whole range of camshafts for 155s; have a search for their name with Google. I think that their importer into this country is EB Spares. Poweralfa in Daventry can supply and also fit them.
I hadn't heard that one about running in cams for 15 mins at 2000/3000 rpm. What does it actually do for the cams/followers?
Re hotter cams: the generally accepted mod for the GTV6 2.5 is to put in cams from the 164S. Until the friendly people at AR Ricambi upped the price by something ridiculous like 200% or so. But in the GTV6 they give 12-15bhp over the standard 155bhp. Dunno about the 155V6 though.