Electrical – How to attach a 3.5“ mounting plate to a 2.75” round electrical box

Your fixtures should have a maximum thermal power rating, as long as you don't exceed that with your bulbs, there should be no safety problem from the fixture.

If you were to use light bulbs with electronics such as CFL or LED, you would also need to insure you don't exceed their temperature ratings. CFLs, for example, are usually made to be used in a "base down" position so the heat will rise and dissipate. A CFL used in a recessed fixture will be in a "base up" situation where the electronics will be hotter.

Have you considered using a PAR E27? I don't know your fitting size but a PAR20, 30, or 38 will probably work out better than trying to adapt a GU10.

Here in Australia you'd probably use a surface mounting block over the top and then an IP rated outlet.

Mounting block is bigger than a round j-box. May need to seal between block and wall and then you're done.

Along these lines:

Source: http://updates.clipsal.com/assets/images/products/A00047211.PNG

Not sure whats available and standards in the US but if surface mounted over the top and larger, shouldn't be an issue but uglier than flushed.

May be able to drill/chase in a large wall box, however it's quite a bit of work.