We already know the answer to this question: It'll perform worse. Apple is developing their own connectivity solution purely to cut costs, not to increase performance. That's why they plan on introducing it in the low-end iPhone, too.
The old Intel modem team, which Apple acquired, never even shipped a working 5G modem. Qualcomm had three generations of product in market by that point. We know how this is going to end... same way it did on the iPhone X, which sourced Qualcomm modems for the Verizon / Sprint iPhone SKUs, and Intel modems for the AT&T / T-Mobile SKUs. The Intel modems performed worse, and Apple artificially limited performance of the Qualcomm modem in order to achieve parity.
This is Tim Cook's Apple. Squeezing the supply chain to wring the most profit out of existing product lines. Very little real product innovation... just iterative designs and reducing costs, wherever possible.