Phex doesn't support Gnutella2, since there is no such thing as Gnutella2.
What you're referring to is Mikes Protocol or the Shareaza Protocol, which is in no way a followup on Gnutella, but a completely different network.
Sadly its performance can't be judged nicely, since Shareaza and the other clients who support it all have a quite substandard Gnutella implementation.
But Phex also doesn't support the Shareaza Protocol, since we believe it best to
stick to one network to avoid wasting bandwidth.
And even though you might have heard other things, there are quite good reasons, why
Gnutella scales quite well.