Unlike the "aggressive" (AG) samples in string sections, all the solo strings have a gentle attack.
Fast sixteenths are "mushy" and too soft, espacially slurred ones, and a velocity-activated accent on a longer note just doesn't sound. Have I missed something?
All I could find was to put a sforzando on every note!
GPO4, Sibelius 6.2, W7.
PS: Miroslav has "bright" attacks available, but not switchable.
Not being a Sibelius user I'm not sure that I can help much, but when I want a good attack on a note I use a key switch to change the bow direction. This really does mean thinking like a string player and using down bow for a strong attack and up bow for a lesser one. Or using the alternate bow key switch on fast passages.