The k frame by itself does not help a car handle any better at all. Zero. All it does is support other parts of the suspension and the engine. It all depends on what kind of parts you bolt ON those different style k frames as to what type of handling you end up with.
An aftermarket style k frame certainly can be made with a suspension that can handle much better than the factory equipment, but remember, it's all about a matched package, very similar to an engine build. Using an aftermarket k frame and retaining torsion bars and stock control arms for example, may not net much of a handling difference from a well thought out stock system. Yet, I've seen people do it.
Just like a well thought out stock style system might out handle the pants off of something that's aftermarket but not well put together. Don't forget that the stock style suspension systems in these cars have won literally ALL kinds of motorsports events through history. It's harder to improve upon the factory system than you might think.
If I was going with the HDK setup for example, I'd go all the way with it. I'd use his control arms, coil overs, whatever brakes and sway bars he recommended. It's a complete package. That's how you'll see the best results, IMO.