Welcome vickg2003,
The 400 should be fine for solo higher speed riding (55-70). I have been on some group rides with my 650 riding 2 up, and we were fine. The performance on the 400 solo, is simular to 650 2-up. The smaller tires mainly mean, they will wear faster than larger ones, and the ride will be a bit rougher than standard motorcycle tires. I haven't heard any complaints on the ride.
The only other consideration would be crosswind. The 400 is more prone to it (buffiting) than let's say the 650, mainly because of the weight.
The 400 should be fine for solo higher speed riding (55-70). I have been on some group rides with my 650 riding 2 up, and we were fine. The performance on the 400 solo, is simular to 650 2-up. The smaller tires mainly mean, they will wear faster than larger ones, and the ride will be a bit rougher than standard motorcycle tires. I haven't heard any complaints on the ride.
The only other consideration would be crosswind. The 400 is more prone to it (buffiting) than let's say the 650, mainly because of the weight.