There are some OTRs that have 450 CFMs - you'll have to search for them.

Can anyone recommend a brand of otr microwave that has a good exhaust system? Thanks.

There's not been any responses to your request because unfortunately no OTR MW has a good exhaust system. As you say, you can only do an OTR MW, so you'll just need to accept the poor exhaust performance. As to your request for brand names, most OTR MWs are made by Sharp and LG, and then rebadged, so you're not going to find much difference in performance among brands. I would just look for the ones that have the highest wattage (1000 or better), since they'll be the most efficient at heating your food.

The overwhelming majority of OTR MW have 300 cfms. A few have cfm's in the 200's, so avoid those. Although Weissman mentioned 450, as far as I know only Jenn-Air makes a MW with 450 cfms, and it is for a 36" range. I think Jenn-Aire "finesses" the 450 number because those particular MW's are for 36" ranges, so the extra width provides the extra cfms.

Frigidaire is the only company I know of that claims OTR MW's with more than 300 cfms. They say their OTRs have 350. I believe Sharp makes the Frigidaire microwaves, and the Sharp microwaves only go to 300, so I don't know how Frigidaire is getting the extra 50 cfms. In any case, whatever you do, AVOID the Frigidaire OTR MW's that are designed with the controls horizontally across the bottom (rather than vertically on the right-side). Those controls buzz out easily, being so close the the heat of the range--really, you'll be replacing it within a year; I don't who the heck thought of that dumb design idea. Frigidaire has a number of OTR MWs with vertical controls on the right side, and with 350 cfms, so check those out.

Here is a link that might be useful: OTR MWs at Abt - You Can Sort by Brand Name

Found another one! It's from Samsung, and it has 400 cfms. However, I am a little concerned with the vent holes across the top--would that damage cabinetry above it? I don't know.

Here is a link that might be useful: Samsung OTR MW with 400 cfms

We have this microhood and the vent do no damage at all to the cabinets. It's been in for two and a half years..... It is indeed about 400 or so and does move a lot of air. Ours is vented right out the back through the wall and the back door is there and you walk past where it comes out to get into the back yard. It is higher than head level, of course but it blows you to death walking past. Hope this helps. We love the thing. No it isn't as quiet or quite as great as our previous hood only but to have the additional counter space an above the stove micro affords, it's worth it....

By definition, OTRs are not good vent hoods. The front burners get virtually no coverage. It is better than nothing as long as you vent them outside and never recirculate, but don't expect much.

I had a GE in my former home. This OTR was available with an option. This OTR had a glass shelf which stored on a track beneath the OTR. I just pulled it straight out and then it dropped down and covered the 2 front burners. Think a Sneeze Guard at a salad bar. It would direct all of the steam and grease to the back so it would exhaust outside. I would fog up at times when cooking pasta but I didn't think that this was any big deal. If I were having a dinner party I would just slide it back in the track, dirty and the next day I would just boil a pot of water and let the steam do it's work and soften any grease. The clean up was a snap. I have never seen one of these since. This was installed 15 years ago.

KitchenAid makes a deeper OTR. I believe they call theirs TruCapture. Check the KitchenAid website for model KHMS2050SWH. Although it is only 300 CFM, it extends farther over the cooking surface, allowing it to capture more of the smoke, grease, steam from the front two burners.

I think I've got the correct link below.

Here is a link that might be useful: KA OTR

Be aware of any head bumping potentialities if you choose an OTR deeper than normal. They are usually mounted much lower than an equivalent vent hood, and many people have problems with those mounted at the "correct" height. You can't mount an OTR at a vent hood height or you'd never be able to access it. So, you have a "meet in the middle" situation where you get some features of one thing and some of another, but never the best of either. An OTR is always a compromise. You just have to decide which feature you can compromise the least.