For our manual overclocking tests, we used MSI Afterburner. Our best results are as below.
With a 260MHz offset, the RTX 4090 saw its average operating clock speed increase from 2683MHz at stock, up to 2971MHz. That was measured over a 30-minute stress test in Metro Exodus Enhanced Edition, but in other 4K workloads, we did see 3015MHz on the core.
The benefits of this overclock varied slightly depending on the workload. Time Spy Extreme and Cyberpunk 2077 only saw 7% and 5% improvements, respectively, but Marvel's Spider-Man Remastered saw FPS increase by 10%, while Resident Evil Village netted 8% gains. Either way, we're not talking massive differences, but we look forward to seeing what can be achieved with a third-party card.