Salah has 14 and 15 league goals over the past two seasons for Roma, Mahrez has 17 and 6. That doesn't necessarily mean Salah is the better finisher, in fact you're probably right, it's the sheer volume of attacks that really matters here. But overall I think it does mean Salah is the better player. Dude is incredibly fast, is a constant threat and can still provide a goal or an assist when he's not playing well.
People tend to exaggerate Salah's wastefulness, and is more a forward then Mahrez is (closer to all out creative/shooting winger then going into box alot), but I defenitely seen Salah miss like half a dozen shots infront of goalie in some games that Mahrez doesnt. Tho as said, he is so quick and tricky he just gets into so many chances. Plus Spalletti plays so hyper offensive that Salah is given the reign and space so much more, then a bunkering Leiceister.
And the comparison is for Mahrez in his best season, last season was fairly average, but at his best, he didnt have many chances but the ones he did he just clinically finished, was kinda freaky how smooth he was.