We have definitely seen excessive wear in some LS7 exhaust guides. Mostly coming from engines with over 30k on the clock. We have seen some heads show inconsistent wear requiring only one or two guides needing replacement. It appears that the sodium exhaut valves are transmitting an excessive amount of heat from the stem to the guide. In some cases the oil is BBQ'd in the guide and the lack of lubrication and cooling from the oil causes the excessive wear (mostly in the lower half of the exhaut guide). We've measured some guides as having over .004" of wear in the lower 1" of exhaust guide length. Porsche 911's suffer the same type of related guide wear on the exhaust side.
The next item to note regarding the sodium exhaust valves relates to the wall thickness of the sodium exhaust valve stem. It's only .040" thick and makes for a fragile exhaust valve. GM uses a good quality steel material but the design spec. renders the valves brittle at the neck. Hence the reason we choose the replace the factory units with stainless or inconel. The exhaust valves we use are manufactured with a .001" oversized stem size which allows us to hone the exhaust guides and remove most or all of the taper in the bore. In cases with excessive wear we replace the guides with bronze.

I can't comment on weather the excessive guide wear is causing the valves to rattle around in the guide on on the seat causing the head failures. It may be due to excessive heat buildup from the restricted heat flow path to the guide. As I mentioned before, there's only 1mm of wall thickness at the valve neck. Not a good situation in engines producing high specific horespower. The stainless exhaust valves don't show the rapid wear that the sodium valves have.

Given the same conditions, the bronze guides will wear as well. The size of the small heat path must be increased. With a solid stem valve heat is allowed to travel further up the length of the stem. It provides a larger flow path for heat dissipation.
The primary heat path is through the valve seat. The secondary heat path is through the valve guide. Lean stoic. conditions at cruising speeds generate lots of heat on the exhaust valves with unleaded fuels. The heat bakes the small amounts of oil that makes it to the lower portion of the exhaust guide. Once the lubrication is gone there is metal to metal contact and on a molecular level electron sharing occurs. The steel molecues of the exhaust valve stem have a greater resistance to wear. The powdered metal guides have mixture of wear resisting alloys along with lubritious properties pressed into the matrix. You don't have to beat on the engine to experience valve guide wear.
The primarly reason we use bronze valve guides is because GM will not sell the guides separately. Rather they will sell you an entire replacement head. The aftermarket parts supply only furnishes bronze alloy valve guides. I personally like the powdered metal valve guides the factory uses but the supplier does not sell piece meal to the public. Additionally we have no long term wear issues with the bronze guides and solid stem stainless steel valves.
In the last decade GM has done a masterful job improving both metalurgy and rocker arm geometry to reduce overall valve guide wear. Rocker arm desings are also superbly executed. The issues seem to only be related to the sodium valves.

The NexTek valve uses XH-428 stainless steel and a completely new manufacturing technique to create a valve with a hollow stem and head. The hollow sections of the valve are filled with sodium. The sodium provides an internal cooling mechanism for the valve, drawing heat away from the valve head.

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