Don't know if it's been mentioned but make sure the is nothing under the seat that could possibly obstruct the airbox snorkle and also take off the airbox lid and make sure nothing has got sucked into the filter and is obstructing the air intake. I had a similar issue although in all gears. When I was riding the bike it would be fine until ~6000rpm but after then it would bog down and stutter.
When I bought the bike I asked the garage to fit an optimate charging lead under the seat which they did but they left on the large plastic instruction label which was near the air intake. When I accelerated the suction from the air intake pulled the label towards it and around 6000rpm there was sufficient suction that the label was flat against the intake and blocked most of the air getting to the carbs. Removing the label and making sure the airway was clear resolved the issue instantly.
When I bought the bike I asked the garage to fit an optimate charging lead under the seat which they did but they left on the large plastic instruction label which was near the air intake. When I accelerated the suction from the air intake pulled the label towards it and around 6000rpm there was sufficient suction that the label was flat against the intake and blocked most of the air getting to the carbs. Removing the label and making sure the airway was clear resolved the issue instantly.