well 6th gear for me has hardly any excelleration also and i only use it to hold the speed i was doing in 5th just at lower rpm, when ever thing get changed on 2 strokes especially intake/exhaust it is a good idea to get it properly jetted to get the benifits.
im lucky with mine it has been derestricted with full dep and snorkel removed and the spark plug is a nice light brown colour!!
chuck in a new plug and run it for a few mile then have a look at the colour see how she is running, but the mixture is crucial to these 2 strokes you dont want her going wrong on you.
merry christmas
ken