On the earlier v6 SHOs, the manual recommends performing a valve lash adjustment every 60k. Some of the later v6 SHO manuals state that a valve lash is only required every 100k, here's the scoop from one of the better SHO mechanics around - Doug Lewis of FPS Automotive :
My recommendation is to have the valves adjusted at 60K regardless of the trans. The reason that the timing belt can 100K on the ATX cars is the way it is tensioned. The ATX cars have hydrolicly dampened, spring loaded tensioner. It does not use engine oil pressure. It is a self contained unit that is simallar to a hood strut. Over time and miles, the belt will strech. The tensioner on the ATX cars will make up for the slack and keep the belt tight. The MTX cars have a mechanical tensioner that is loaded and then locked into place. Once this is done, the belt tension is set and can not move. Over time and miles, the belt streches, but the tensioner remains in the same place. This causes the belt to get loose. A loose timing belt kills the power of the engine because it allows the cam timing to change with the accelerater. When you get on the gas the cam timing is retarded and when you get off the gas the cam timing is advanced. Exactly opposite of what it should do to make power.
In my opinion, both cars should have the valves adjusted at 60K. Ihaven't found very many that are out of adjustment, but I have found wear on the shims. Anything from pits to scratches. Since the cam lobes ride/slides directly on the shim surface, it is very important that the surface be smooth and free of defects. If a pit or a scratch opens up on the shim, it will wear the cam lobe prematurely. The shims are hardened to on grade lower than the cam lobes. This allow the shim to wear before the cam. The shims are much less expencive to service than the cams. When the valves are adjusted, it is VERY important to remove each shim and inspect it for pits or scratches. A valve adjustment is much more than using a feeler gauge. The clearance might be within tolerance, but the shim could be pitted and tearing up your cam lobes.