Your stock mass air will support more power then your engine in n/a form will handle. I've seen plenty of stock 4.0 SOHC mass air's running 190-200rwhp and they even use the SOHC mass air on the Explorer Express kit, I've pushed 230rwhp using it there before it finally pegged.
Also, it's alot easier to tune to, the Pro-M mass air squew(sp?) the transfer readings and therefore can cause alot of headache's from a tuning perspective. If I were to recommend a larger mass air for someone who needed one I would say to upgrade to a 80mm from a GT or F150 but stick with a Factory Ford meter because the electronics are soo much better.
If you want a n/a SOHC at it's highest then get the JBA headers, a good air intake, cat-back exhaust system and random technology's Y-pipe. Also drop back to a AGSF22 or AGSF24 spark plug depending on your year model SOHC and gap it around .045-.050. Finally not really for power but for dependability get some JBA plug wires. Round it all off with a good SCT chip/tuner like that from my company,
www.bamachips.com, and you'll be around the most your gonna get without doing any internal or forced induction mods.
Later Doug