Strike foils on all ships are supposed to releave heat stress generally... That's what I've read and makes a bit of sense. On the B and X wing fighters there's cannons on the ends... Y and A Wings have fewer cannons overall, so perhaps the heat issue isn't as big an issue with them?
Or it could not be linked to the cannons at all... Though gaming shows the engines, shields, and lasers are all interlinked energy systems on ships (the "ELS") and thus one affects the other... During the civil war's beaming technology developments those too interlink and advance the productivity of the ELS(B) then on small ships. Probably in some similar but smaller fashion to what destroyers and capital ships in general have.
I can buy the whole heat disipation thing though for "S-Foils" on really anything (not just fighters). If we're talking maneuverability though, I dont' think that S-Foils have much impact... When you look at any SW fighter, none of them appear aerodynamic in the slightest. The way they fly in atmosphere I believe probably is mechanics more than design. They can obviously vertically take-off as evidenced on Yavin IV, so I think they likely stay in the air more via their "technology" than anything else... Maneuverability and such then probably isn't much different than in space I'd think. Thus that lets the Rebel Speeders be the superior weapon on the field for certain circumstances... They are more built for the atmospheric flight while the space-capable craft are capable of atmospheric flight but sometimes there needs to be something specific for it.
On another note, I've never liked the whole attempt to tie the ARC-170 to the X-Wing... While there's vague physical similarities, the EU shows there's also dozens other fighter types by dozens other manufacturers (including Incom) that bear the same "look" and shape... And many of them are closer in purpose/style than the ARC-170. For me it's a matter of Lucas overdoing it on trying to tie one trilogy to the other...
The ARC-170's S-Foils I don't see as being exclusive to that design then. Many ships by many companies can feature swing-wing designs for similar reasons, or maybe other functional purposes.
The ARC-170's 3-man crew, weaponry, and other features really imply it has a purpose wholely unique of that of the X-Wing fighter from the classic trilogy... I don't see it as much of a precursor to the XW so much as it being just (perhaps) made by the same manufacturer... Perhaps they realized the advantage of heat disipation and wanted to incorporate that in the XW then or whatnot. I however dont' see a direct evolutionary link between the two fighters. I stick with the EU line that the Z-95 Headhunter was the real grandfather of the T-65, and that the ARC-170's a ship unto itself with no (known) future in Star Wars.