How much do you allow mages to pump up their missile spells when using the default GURPS magic-as-skills system? Particularly with casting level 15+ (enough to get a discount) and a few levels of Magery? The GURPS FAQ 4.3.3 here mentions that the total cost of the full charging time is discounted once - so at casting level 15 you can charge a 1 die fireball for 1 second for free, but you cannot cast a free 3 dice fireball by spending 3 seconds charging it - that would instead cost 2 energy points.
But what about the upper end? GURPS Basic Set page 240 says you can “invest one or more points of energy in the spell, to a maximum number of energy points equal to your Magery level”. Suppose a PC with Fireball-15 and Magery 2 spends 1 second creating a 2 dice fireball. This has cost them 1 energy point. Does this mean they could instead spent 1 second investing 2 energy points (their maximum Magery) into a 3 dice (discounted price) fireball? I saw this idea posted on the GURPS wiki here, but I haven’t found anything discussing this idea elsewhere.
Peter V. Dell'Orto has mentioned that even allowing the discount to apply to each charging turn hasn't wrecked his game, so I'm not worried about the balance component of 1-2 extra dice of damage; I'm just curious what other GMs would permit at their table.