My current pack is a 3# suspended mesh back panel MYOG 50L of spectra gridstop that weighs around 4 oz/yd with the coating. It has held up very well for years, so don't see the point of doing one with 5 oz Xpac. That is going in the wrong direction, weightwise, IMO.
Could cut the weight by at least half by going to either 1.5 oz cuben or sil-coated 1.9 oz nylon, and building a lighter frame out of carbon or FG rod to suspend a mesh back panel. Not sure that the cuben would be worth the greater cost, and being an old fuddy-duddy, the feel of nylon cloth appeals to me more if it is not that much heavier.
For those needing a bomb proof pack for trail work, climbing or whatever, the heavier Xpac probably makes sense. For backpacking trails, or off-trail in open country, the lighter pack will be fine, IMO, and contribute greatly to reducing overall weight.
For sake of comfort, I am willing to suffer the additional weight of a suspended mesh frame, but that is apart from the fabric issue raised by your thread.