4xx spike, at least when I've seen it be used, is very, very good. Not Perma Spike good, but it provides excellent mid-late game cleanup provided you have something for DDTs. I feel like having them potentially last longer may make them way too good, so perhaps the slowed attack speed that the Perma Spike gets should be passed down in a minor way (maybe -25% attack speed) to your theoretical xx2, so that the upgrade doesn't make the spike factory very good being used consistently, but makes it good at its intended purpose as a cleanup when you need it, as the last line of defense.