It's a little difficult to understand your problem without any audio reference.
I just can post another example, this time the clarinet also using extensively performance legato patches.
http://vsl.co.at/Player2.aspx?Lang=1&DemoID=4650
All this solo instrument demos were performed live on a keyboard without any editing.
You are right that there are differences between a slow and a fast legato movement,
that's why we offer normal and fast legato performances (and also the performance trill patches, which also work very well for fast legato performances)
Also our speed controller helps to automate the switch between slow and fast passages
Just another few things which are important to know:
- perf-legato patches are monophonic,
- you have to perform legato (there shouldn't be breaks between the notes)
- it could be tricky if you perform a legato line changing from piano to forte (during legato)
here the transition between the notes gets obvious
In this case it is better to activate velocity crossfade, and make a fast crossfade change from low to high.
best
Herb