RE: Estimating speed??
First off, it was not a F1 airplane that killed the guy. No one was ever killed by an Formula One airplane, partly due to the tight controls on airplanes that the rules imposed.
The guy was killed by a "Warbird" racing airplane, where in my opinion had few rules and very poor judgement in the event organization and race operation. The guy in question was leaning against the cage, so it offered no protection. This style of metal cage would stretch about 12-18" when impacted by an F1 airplane at speed, so the secret to surviving the impact was to be away from the walls by more than that amount.