This may have been obvious to everyone else, but it only just struck me that a version of Pilum of Fire that had Sight range instead of Voice would allow a magus to shoot a jet of fire from their hands extending all the way to the horizon, automatically hitting a target many many miles distant, for only a single increase in magnitude.
Unless I'm missing something? 
I assume that the issue is the loss of penetration by the increased level?
Bob
You are not missing anything. The Flambeau in my current saga (and the Flambeau played by another of our friends in a previous saga) has created Sight range versions of Pilum of Fire and Ball of Abysmal Flame and is studying spell mastery for those.
The main spells from the books are frequently reinvented with slightly different ranges, targets or durations as needed.
And if you have Flexible Formulaic Magic, you don’t even need to reinvent the spell!
«Many many miles distant» is probably an exaggeration. Making out the person you want to hit a mile away is not easy. Yes, I agree, several miles is possible in certain cases, but «many many»?
«from their hands» may also be an exaggeration, but the origin of the jet is only cosmetics. The real effect manifests at the target. The cosmetics of the jet flying some dozens of paces is nice and appropriate. The jet running a mile or two in 6s (one round) ... well, maybe, I am not convinced.
But no, I agree with the others. You have not missed anything.
Take two magnitudes extra, and you can hit through an arcane connection across the continent ...
Yes, arcane connection seems much less bizarre to me. It's the idea of going from shooting fire at someone within speaking distance to shooting a jet of fire multiple miles in length soaring across the countryside that I found striking. If the cosmetic of the jet was removed, and the magus simply pointed at a distant person who erupted into flame, I wouldn't find it so odd.
The jet won't be many miles in length.
The spell description says it is a 2-foot jet that shoots forth - so most of the time it will not extend the entire length between caster and target.
If the jet of fire is to hit the target in the same round it is cast, then it will barely be visible when traveling several miles - it would have to travel far too fast through the air to be easily seen.
I would think a target has to be clearly distinguishable. If your magi can look at the horizon and determine that dot is a person, go for it, otherwise, I would argue they can't be targeted. While horizon seems extreme, going beyond voice is very useful, especially if you are taking on larger than human targets, so the useful range would be even further.
With a little bit of magic - such as the spell Eyes of the Eagle - it shouldn't be hard to distinguish a target even at a very great distance.
Fair enough. I think it is implicit that the cosmetics has to be reinvented when you change the parameters. Spell design mechanics apply only to effects, not to cosmetics.
You just made me notice the comma in the description for the first time. I was reading it as "a 2-foot thick spear-shaped jet" of whatever length required to reach the target, but now I see it actually says "a 2-foot, thick, spear-shaped jet" implying the 2 foot measure is actually the length.
Yes this seems a fair interpretation. The guidelines are intended to indicate what is possible, but not how it is achieved, which has to be worked out depending on the overall parameters and context.