> but does anyone actually do that?

Backtesting a regular single symbol model on CPU is often not cheap, especially at higher frequencies. Now multiply that by 50 correlated instruments, each having say, 1500-3000 strikes+sides+expiries in their respective option markets, and the value of GPU offload should become obvious. Backtesting dispersion trades on the S&P 500 using just 5 strikes per underlying, or any other scenario where the evolution of the volatility surface is important would already potentially involve >25,000 individual option markets

All professionals backtest, often numerous times per day