The engine has reached it's limit when you have the pedal pressed to the floor and you're not getting anything more out of it. As I've said before in another thread, you can't harm a warmed-up engine by flooring it. The engine will self-limit its speed if necessary. They have rev limiters and the automatic transmission won't shift to a gear that will overspeed the engine. Try it. Shift to S mode and downshift a couple times. It won't let the transmission downshift if the next shift would put the engine past redline, even if you've requested that gear. It'll wait to shift until the engine has slowed down enough that the shift won't cause the engine to overspeed.I was one of the whiners (about Sienna being underpowered.) I was hard wired to assume that loud engine noise means it has reached its limit. Over the time, I learned to ignore the droning noise and keep going at the peddle to get as fast as I want. Since then, I say I have passed 500 cars for every one that passed me, including on high grade uphills ( Ex. Vail pass.)
The only way you can overspeed an engine is with a manual transmission vehicle, and even that requires downshifting too much, because even if you don't upshift before redline, the engine will hit the rev limit and will limit its speed. Even my motorcycle with carbs has a rev limiter, which since it's carbureted with mechanical throttle can't work by simply closing the throttle like a vehicle with electronic throttle can. So instead they cut the spark plugs to limit engine speed.
When I was young, and dumb, I found the rev limiter in the 1991 Honda Accord I was driving at the time. This had a mechanical throttle, so like my motorcycle it worked by intermittently cutting the spark plugs. It took until 114 MPH (indicated) to reach the rev limiter...
So yeah. If you want the acceleration/speed, and the engine is warm, ignore the noise. That's normal, engines make noise. Ignore it and press the pedal to the floor till you've reached the speed you want. The engine will be just fine.