I agree with everything you said A. P. The saw condition and chain setup would be the determining factors. If those variables could be eliminated (dificult), then it'd be very close. The outcome also would depend on which exact saw models you were comparing. With Meatsaw's original question ('regular' 10-10 vs SXL-AO), I think the 10-10 would just edge the SXL out in smaller wood, while the SXL would just edge out the 10-10 in larger wood. In my experience, the 10-10 has more top end, while the SXL has a tad more grunt. Both are excelent saws.
An XL-12 is also around 54cc, so is a closer match to a 10-10 in that regard. There were two versions of the 54cc XL-12 made. Those with the 'flat' reed, and those that had the 'pyramid reed' setup from the SXL. I've never seen a pyramid reed equipped XL-12 in person, but I have an IPL that lists XL-12's "with pyramid reed system" along with SXL-AO's. It's a later IPL. The non pyramid reed XL-12 sas (and the 58cc Super XL-12 saws, which also have a flat reed) run out of steam on the high end when compared to an SXL-AO (or I'd presume an XL-12) with a pyramid reed setup. It's quite noticible. For that reason, I'd say a 'standard' 10-10 woud best a non pyramid reed XL-12 without problem. A pyramid reed equipped XL-12 would be much closer.
The XL-12/SXL-AO saws are lighter and more handy than a 10-10 in my opinion. They also have a much less iritating sound and the vibes are less bothersome to me than the 10-10 (both very subjective I admit). I prefer running the Homelites to the 54cc 10-10 saws. I've never ran a 57cc 10-10S so I can't comment on how they'd stack up against an SXL-AO. I just have ran the 54cc 10-10 saws and the larger 70cc variants. The 70cc McCulloch 10-series saws "sound better" and the vibes "feel better" than the 54cc 10-series saws to me. Again............very subjective. The 70cc 10-series saws also don't weigh much (if any) more than the 54/57cc versions, yet have significantly more power and are just as reliable. For me, I choose the Homelites for the 60cc class work (with 16"-20" bars) and the McCullochs for the 70cc class stuff (with 20"-24" bars). The 70cc+ versions of the Homelite saws (XL400A and XL500A) weren't successful. They had a tendancy to beat themselves to death from what I've gathered from old saw shops and other folks that have ran them. I've never even seen a runner of either saw in person. Just a few dead (but still nice looking) XL400A's. Never seen an XL500A at all except for in pictures. I'd like to compare a good running XL400A against my PM700 someday. I'd also like to compare a good running XL400A AND XL500A against a good running Poulan 245A.