And possibly the voltage curve may be different which will screw up the ECU interpretation of the air flow amount. A quick lesson on how the MAF works. The ECU attempts to keep the sensor wire at 200*F above ambient temperature. As more air flows across the sensor it starts to cool and the ECU increases the voltage to bring the sensor temperature back to 200*. If the MAF has a different voltage curve the ECU has no way of knowing this, that's why a tune is needed or having a MAF that sends an adjusted signal to the ECU. These readings trigger other events such as fuel loads and timing. That's why these parts just cannot be changed without consequences. Plain and simple.
I swapped to a 98 gt maf years ago as my stocker was close to pegging and I did not see the need to move up to a 90mm LMAF. That said I also wanted to play around with other non-popular stock mafs with the tweecer tuner. Its not a big issue and really needed only if your peggin the stock meter. I posted a graph in tuning of the maf transfers comparing stock, 98gt, 99gt, LMAF...this was like 4yrs ago or so though so who knows if its still out there.
If your not pegging or close to 4.3-.5-ish volts there is no real need and your not going to peg your stock meter with 19's and the stock longblock unless your running boost (and if your still on 19's in this case you have a ticking time bomb).