Back when I was using thick Dyneema lower shrouds, I'd have said mast bend control was not as good as the original system, but that a gnav was worth it for the extra room for the crew. Keeping the crew weight forward in a 3k is important, and a gnav helps during tacks.
But now I've switched to the wire lower shrouds (as supplied in the kit, I believe, along with a plate for attachment to the mast), I'd say mast control is good enough with a gnav, so a wholehearted recommendation for it.
5mm Dyneema is supposed to be extremely resistant to stretch - more so than 2.5mm wire, but in practice every time Dyneema goes slack it has to be fully stretched again before it becomes stiff enough, which limits its effectiveness, especially in an adjustable system such as I was using.
