So many mistakes, so little time.
Based on five years of reading Mustang tech posts I would say that the basic error most new Mustang owners make is not thinking ahead and just slapping on parts without a real plan.
I would add that the first thing most new owners want to do is get a bigger, louder exhaust...mostly louder. No problem, but as we all know, just removing the cats and installing a pair of Flows will do that just fine...better flow, more noise. Unfortunately, many new Mustangers want to add big-tube headers and pipes on a stock engine, thus minimizing the one big advantage Mustangs have right off the production line: torque. Same with bigger TB's. Chips are another 'mod' that often fails to deliver - especially on a stock engine.
I usually advise members to think through what you want from your car and what the reality is of getting there. Saying:
"I wanna go 11's on street tires but I need good gas mileage cuz I hafta commute 100 miles each day and I don't wanna lose any drivability...and I only have about $500. to spend on mods right now but maybe in a year or two I can spend more" isn't going to cut it.
Don't laugh too hard...I've seem very similar posts right on this forum.
The best thing a new Mustang owner can do is a thorough tune-up, a K&N, rear gears, subs, maybe a shifter (5-speed) and then develop a realistic plan for where they want to car to be when it's 'done' and go from there. That takes planning and patience, none of which are very exciting on a Friday night cruise when you're surrounded by well-modded Mustangs and F-bodies with tons of goodies under the hood that can smoke you easily. Been there, done that.
When a newbie reads the ads in the Mustang magazines that all promise 5 - 10 - 25 HP gains it's tempting to run out and buy shiny new parts and start slapping them on, then go for a run and (hopefully) 'feel' the difference in the butt-dyno, often to be dissappointed. The truth is that a lot of these advertised HP gains are on fully built engines with the stock part left on so that when they install the aftermarket chip, intake, whatever, the HP jumps up.
There is no real secret to better performance. It takes money, of course, and planning. Any 5.0 can run low 14's and 13's are easily attainable for many 'Stangs - 5.0 and 4.6 versions - but beyond that, you need to get smart and spend some cash...usually on heads and then matching intake, exhaust, fuel and spark delivery. Control arms, subs and the right rubber are a big factor too as is learning how to drive your 'Stang to extract the maximum performance from it.
Done right, you'll have a 12-second 'Stang and be ready for most anything on the street. Done wrong, you'll tear your hair out and waste a ton of money trying to beat the guy who had a plan, some patience and did it right.