I had a similar question actually, we currently send the entire revenue (i.e. $100 of Gross IAP, are sent as $100) however have wondered if I am better off only sending 70% of that amount. With a move towards automated campaign optimisation, I wonder if sending the gross amounts possibly gives FB and other networks a "false positive" on the ROAS front in some circumstances where you may have made a slight positive gain, but not enough to offset the store fee.
Spend $100 on FB ads
Send FB Events with $120 in Revenue (Gross, before store fees)
FB Reports as +20% ROAS and their algorithm theoretically deems it a success.
Reality is, after taking off store fees you have only made $84, so actually negative ROAS relative to the original $100 cost.
The alternate is, we only send $84 (the net amount) of revenue to begin with, and FB now has the signal that the campaign is not yet ROAS positive and either optimises accordingly, or just ends up failing to deliver enough volume as the ROAS goal is too aggressive for that particular product.