Speaking as one with plenty of track time but far less ability who has run both 5 and 6 speed boxes on a 620, there are more tracks where the 6 speed was a benefit.
The lower 1st gear helps on starts, and there are tracks where you can gear to use 6th only when you have to - ie its dry and you have time and space to use the fastest line rather than a defensive line.
With a 5-speeder you just don't get that option.
Oh, and there is another thread of mine which tells which 'boxes fit, from practical experience and use of the parts catalog.
But what do I know as an ex-racer......
maybe
Paul
15th year in DD #68 and getting slower by the year
1199, SS800 & 620SSie DD racer 
paynep
Linear Mode
