wdba, mine had 12°, but I had no damage due to snapped rails since mine are the old-style metal-backed aluminum ones. I'm sure, if I had plastic rails, it would have snapped. My heads indeed had telltell signs of slap.

Not sure about possible vac sources for the SL, but on my 4.5, there's only 1 line to the distributor (vac retard for idle). Other vac lines not connected to the engine operation but run off manifold vac are the central locks, tranny, brake booster, and MAP sensor. These, as well as the intake manifold seals, are areas where vacuum leaks are possible. Measure your vacuum at idle - it might be anywhere from 14" to 22" at idle depending on the idle air speed screw setting, with NO LEAKS! You want it to be closer to 22", so you may have to close the air speed screw a bit to do this and compensate slow idle with the ECU idle mixture adjustment screw. I've had lumpy idle on my 4.5 before, and depending on how lumpy yours is, it may be just the air speed screw and idle mixture screw need to be synched. If you adjust the one and not the other, you may get a lumpy idle. The idle mixture screw doesn't affect the idle as much until the engine is warmed up, really, because of the function of the AAV.

FWIW, I keep my idle air speed screw open a bit further to keep the immensely large manifold vac (resulting from letting off the gas completely at high speeds) from sucking up tranny fluid and causing billowing white smoke.