My dog needs to lose 2.5 lbs. Should I cut food or use a diet formula?

You are correct that it is difficult to find a good quality weight loss diet. Good quality meats and fats are high in calories. At the end of the day, weight loss is about calories. So, a weight loss food is beneficial because they can eat the same amount of food. But you can achieve the same decrease in calories by feeding less of the Acana that you are currently feeding. A 10-20% decrease in daily calories is exactly what I recommend for weight loss, so you could simply decrease from a level 2/3 cup to heaping 1/2 cup for a month. If Chloe is losing weight, then continue at that amount. If she is not losing weight, then decrease to a level 1/2 cup. Since she is a small dog, it should take her a number of months to lose the weight, so don't be discouraged if she only loses a few ounces in that first month.
Good luck! I hope that was helpful. Thanks for using Petco Pet Education Center, formerly Petcoach.