Extended length basically adds a lot of swing weight to your frame.
If you can handle the extra swing weight, you will get that much more power. If you can't handle it, it will slow down your racquethead speed tremendously.
The standard length is 27 in. Most people would not have any problem swinging a 27.25 in frame. 27.5 would be a bit more challenge but if you stick to it, you will get it down eventually.
Personally, the biggest challenge for me was the 28 inch. I was already used to wielding a 13-oz 27.5 inch frame. When I moved up to 28 inches, I felt the added leverage and power on my flat serve, slice, and the two-handed backhand felt fantastic. The forehand was a wash. I got a bit more power but I also had a bit more difficulty with body shots.
But it did not work for me on my kick serves. You see, I suffered a rotator cuff injury before, and my shoulder is not as strong on hitting up kick serves as it is on hitting more forward on flat serves. As a result, the kick serve became a huge liability.
I tried many different longbody racquets, but at 28 inches, I needed the racquet to be 11.8 oz or lighter for me to hit effective kick serves. A huge difference from the 13-oz frame that I could comfortably wield at 27.5 inches.
The same will be for your 1-handed backhand. If you have really good racquethead speed, the extended length might work for you. If not, it will work against you, and you will find yoruself hitting a lot of balls late.
It is all relative. It depends on the length of the racquet, the weight of the racquet, and your skills and strength.
