Offiah went round players in a way I've never seen before or since - one game vs Widnes he took the ball about 20m in and literally ran around the Leeds backs on our right to score (including John Bentley, who was no slouch). I'd say it was his elusiveness at speed but also his absolute confidence in what he was doing/about to do that made him unique. Right at the tail of his career he played for London vs Leeds in the CC final 1999 (aged 33), way past his best but he scored a try and skinned our fullback with utter ease from a fair way out. What was amazing that the second he got the ball you knew it would be a try despite having to beat the winger and a well-placed fullback.
In terms of pure speed he definitely wasn't the fastest - Phil Ford caught him over a distance at headingley, and he lost that sprint challenge in Australia as well. But there was something about him that made him seem superhumanly quick.
As an aside from that 92 series there's a try scored by Steadman in that 1992 Melbourne test where he showed amazing pace as well. That 92 touring team was easily the best we've sent in the last 40 years.