3/8" extra width on both sides won't be an issue with most cooktops as long as the counter surface is flat and level and the cooktop contacts it evenly all the way around. Wood does tend to warp a bit over time, so double check it in multiple dimensions. Be sure to seal the wood of the fresh cut, and you should be good to go to just drop it in.

If it won't show and anyone has doubts about weight, you can easily "picture frame" the cut out from below to keep the cooktop from shifting.

Cooktops don't weigh much(~35lbs) although things go up when they are loaded.
You can glue and screw the small fillers you need. The front and back will give enough support.
I would have a 'Maxisense' shipped from the UK.
The best quality at the best price.

You would want to ensure that a device designed for 50-Hz mains in the UK will operate without problem on 60-Hz US mains. Many electronics power supply designs can cope with the higher frequency, even if not deliberately so designed, but acceptability should be confirmed from company literature or service desk consultation.

kas

áMuchas gracias, all!

Given that my current and future tenants aren't as appliance-crazed as I, and given that any induction cooktop (not to mention the Gaggenau oven, the serious range hood, etc.) is already a fairly over-the-top amenity in a Brooklyn rental apt, I'm not inclined to jump through extra hoops or make my tenants wait an extra week or two for an extra-super-special model shipped from Europe.

For my next place, though, when I finally manage to pry us out of my wife's Upper Central Side abode and back to Brooklyn (or its PNW cousin, Portland)... then yeah, I'm going that extra mile. Sadly the current place has woefully inadequate electrical capacity for induction, so we're stuck with gas.