You don't need to replace the MAF unless you are pegging it. Pegging is not maximum airflow - it's when the MAF reaches 4.7v on a stock system without a chip. A chip can extend that to 5v however. The MAF tube itself is not a restriction in a system
The BEST MAFs bar none are OEM MAFs, we recommend people keep stock MAFs until they peg them, then upgrage to a 90mm LMAF (Lightning MAF) - good to about 450 RWHP. After that an SCT BA 2400 MAF is good to around 800 RWHP before it pegs.
With LMAFs and SCT MAFs you do need a chip. If you think on a '94 and up you can just get by with slapping on a Pro-M or C & L MAF that has been calibrated to the injector size, you are mistaken - you'll still need a chip. The MAF is used to calculate airflow to the engine and it's also used to establish where load (volumetric efficiency) is. On '94 and up Mustangs, timing is determined by load. Say you get a MAF calibrated for 38# injectors (just a number). When Pro-M 'calibrates' the MAF, they adjust resistors to send less voltage to the EEC. In this example, it's cut in half - which, if it done right is theorectically perfect for adjusting the injector's pulsewidth. The problem is by cutting the voltage in half, the load calcualtion is also cut in half - so instead of the timing (on a T4M0 processor) being 25*, as it is stock, it is 38* at the lower load value. This may or may not be an issue with an NA car - it probably would be - but on a blower car it is VERY BAD. The other thing is the base fuel table is also based on load - and guess what - as load goes down, the fuel curve leans out. So you have more timing and less fuel - a bad combination.