From what I can tell it looks like that model had Dual-Channel AC capable radios with MIMO. The IEEE 802.11AC specification has wiggle room all the way up to 1 Gbps (~1024 Mbps give or take a few baud here and there). Though this is only on the 5 GHz spectrum and using a multi-station setup at 256-QAM Modulation with 8x/8x MIMO streams. The iPad is capable of the 1 Gbps maximum throughput according to spec. Though there’s so many variables when it comes to wireless transmission and integrity that changes on a per second basis that it’s impossible to nail a hard number without controlled environments.
Unfortunately the only answer I can offer is a range. The 877 Mbps sounds like it was definitely using MIMO on 5 GHz. Generally the average data stream on the 5 GHz AC spec is 200 - 500 Mbps, give or take a few buckets worth of megs here and there for data bursting, QoS controls, traffic shaping, environmental changes per second, TCP vs UDP traffic, etcetera etcetera. The higher end of 800+ Mbps is definitely possible from what I’ve seen on recent routers available at the consumer levels but for general use applications that number should be much lower.
Best of luck and have a great day!