HIFU to slim down arms? Yay or Nay? (photos)

I appreciate your concern. In the U.S., Ultherapy is the only approved HIFU device available. While this treatment is excellent for firming the skin on the face, it would not likely provide enough correction for loose arm skin. I would probably recommend ThermiTight - this device delivers radiofrequency energy at a deeper level via tiny probes under the skin. It also melts the fat which will help to slim the diameter of the arm. The ability to heat deeper tissue results in more firming than we can usually achieve with Ultherapy. 
Make sure the doctor you work with is both experienced in these procedures and has the appropriate qualifications to ensure an optimal outcome. 

Nonsurgical options give only mild improvement compared to surgical lifting. Skin tightening can be done with a combination of Vanquish, truSculpt iD, Venus Legacy, Exilis, and Cellutone/Zwave. However for best results, an arm lifting procedure can be combined with a high definition procedure at the same time. I recommend a hi definition procedure like VASER liposuction. I use VASER HI DEF with ThermiRF and fat grafting to get the most fat removal, skin tightening, contour and definition. This combination will give you the most toned result. I recommend getting a formal evaluation with a hi definition physique expert. Best, Dr. Emer