I dno't know what the original thread said but I have done some investigating and have come up with this [I am going on the assumption that the scaled roll will have a -10 total crit number than the non-scaled spell, is that what you all are going with?):
H&S internal only (no comparison to original crit tables): You will get a higher amount of hit damage, though the critical type (A, B, C, etc) will stay the same. This increased damage can end up being quite a bit if you scaled more than once. Is it worthi ti to you? I don't know, that is up to you. In one instance, the results of a spell scaled all the way from tiny to huge was 6L (tiny/100 result) vs 22L (huge/60 result). In the end the difference was 16 hits. Is that worth it for you to have a greater chance of "missing"? That's totally up to you.
H&S Compared to the original charts: No reason to scale. In one example, the original chart ended up with 1 extra hit, 2 more rounds of stun, and an extra point of bleeding each round. Not earth shattering, but better none-the-less.
Original crit charts only: This is the weird one. With the +10 and -10 pretty-much offsetting each other isn't the resulting number basically the same? The only diffeence being the max possible number - which only comes into play if you roll really well. Again, totally a decision for the spellcasting player.
It seems to me that the way it was originally developed (no H&S) - with each size increase granting a +10 to the crit result but also a -10 to the casting roll - that scaling would end up being a totally personal choice. Do you go with the possible bonus damage or not?
Of course, in the above examples, the added PP expenditure meant a slower casting time. That is the same across the board so I decided to not factor it in.
Personally, if/when using H&S I would go ahead and scale the spell at times, usually whan I wanted to do as much hits as I could.