I have AL manifolds & risers (GLM) on my small block. There is a slight temp differential between port & starboard due to the way the water is routed through the block and t-stat housing, at least with an Alpha. At idle they are both cool enough to touch and stay between 120 and 140 on the temp gun; this is with a 140 t-stat. After a hard run the port gets to 140-145 and I can still rest my palm on it. Stbd gets up to 160 and feels like you could fry an egg on it. Just a 15-20 deg difference but it is a pretty big contrast by touch! The temp won't drop that fast after a hard run - you've got a couple minutes to pop the hatch and check things out. Use the temp gun and shoot your whole motor at idle and when hot. Exhaust, temp senders, t-stat housing, oil filter, oil pan, intake manifold, etc. Get a good feel for what your baseline is and then you'll be able to ID an impending cooling problem before it's critical. What temp t-stat are you running and what does the gauge say? Any difference from prior to the exhaust switch?
Bottom line, unless you've got a cooling problem I doubt your risers are getting to 200. If they are you probably need to troubleshoot your cooling system because you're not far from starting to scorch your exh. hoses. The AL is a lot more effective at transferring heat than cast iron and mine have always seemed "hotter" to the touch for whatever reason. Also, make sure you re-torque everything (esp the manifold-riser joint) after a heat cycle or two.
"Speed's fine, but accuracy is final."
- Bill Jordan
1998 Donzi 21LXR
1971 Boston Whaler 13