Is the ViewFinity S7 good for gaming?

Only casually. It runs at 60Hz with no adaptive sync, so fast shooters will look less smooth than on a 144Hz panel. Its strength is sharp 4K for work, not frame rate.

Does it charge a laptop over USB-C?

Most versions include USB-C with power delivery, enough to run and charge a typical ultrabook over one cable. Check the specific model's port list, since some regional variants differ.

Is 27 inches too small for 4K?

No. At 27 inches, 4K gives very sharp text, though you will likely use display scaling around 150 percent. Some prefer 32 inches for 4K, but 27 is fine for desk work.

Does it have built-in speakers?

Some versions do, but they are basic and quiet. Plan on external speakers or headphones for anything beyond system sounds.

Can I mount it on an arm?

Yes. It has a standard VESA mount, and the included stand also offers height, tilt and pivot adjustment.

Is it good for photo editing?

It is reasonable for hobby editing thanks to its IPS panel and 4K detail, but it is not a wide-gamut or hardware-calibrated display. Professionals should look at a color-critical monitor.