360 Modena with Tubi, lack of power and sound

Just purchased a 99 360 Modena, fully serviced, low milage and extremely well maintained. However, having some strange experiences that maybe someone have seen before. The car was fast, willing and very nice and noisy (propper Ferrari sound with clapping exhaust) fitted with Tubi after the cat's. But now the car seems to lack the power, the screaming sound is gone and it just sees a bit dull... I was sure it was something to do with the vacuum or solenoid for the exhaust valves, but the car dosen't have them due to the fitted Tubi.. I am running om E10.95 octane. Could it be just due to this..fill only 98??

 

There is no alarms or any sign of mailfunction...revs normally, just sounds a bit restricted sound...