I set the temp of my deep fryer according to the table that came in the instruction booklet it came with. I tried it once at the highest setting and wasn't happy with the result. High heat, although having its advantages when cooking, isn't always the best way to cook as what you're cooking might be done on the outside sure, but the middle could still be raw.
For example when you're cooking a steak, you could find the outside of your steak looks like its done, but when you cut it, it's not anywhere near the desired doneness you were seeking. Eventually though, through trial and error, and lots of repetition and practice, you learn how long it takes to cook it so it's done to your desired taste, (BR, R, MR, MW, W, bottom of a shoe.) and the best method to get it there whether cooking that steak on a bbq or in a frying pan. If you like your steak R to MR, you don't and most likely won't put it on at the same time as a steak for someone else who likes their steak W, or like the bottom of a shoe and you're likely going to have a slightly thicker cut than they will because it'll take longer to cook theirs to their taste (W), than it will yours to that R or MR you like. When I cut my own steaks (which I'll most likely be doing later this week after I get paid and go get a loin) I always cut my steaks at approx 10 oz, and my wife's at 8oz, because she likes her steak W, and I like mine R to MR, so hers always goes on first because it's not as thick a cut, and it'll cook faster, so when I flip hers the first time, that's when I put mine on and that way they're done at the same time, and cooked the way each of us like.
What you're using too cook also makes a difference. We have a gas stove, my son has an electric. Cooking times will be different because our gas stove cooks faster than his electric because gas cooks hotter than electric elements do. Likewise, cooking a steak in a frying pan on the stove will cook the outside of the steak much faster than cooking it on the BBQ. Smaller surface, but the middle won't be anywhere near what I'm looking for as a finished product, so if I'm cooking a steak inside in a frying pan, I'll start it in the frying pan, then finish it in the oven.
Each fryer is different as well. My current deep fryer is substantially deeper than my last one, and that makes a big difference in how much oil you can put in it, how long it takes to get up to temp, (and then reheat once you've taken the product out if you have more to do) and how much product you can cook at one time and how that product will cook, but you still don't want to overload it during cooking or it's not going to cook evenly which will result in some being overdone, some potentially being raw still in the middle, and that's when you might find you're batter is greasy. If that's the situation, cut back on how much you're cooking at one time, you might be overloading the fryer and the product won't cook evenly.
That said, as I indicated, when I do batter fish and deep fry it, I always just blanche it first and then take it out to sit until just before I'm ready to serve, then I put it back in to finish and when I'm finishing it, I always make sure to use one of the other smaller baskets the fryer came with (1 big, 2 small) to hold the fish completely submerged in the oil so its cooking evenly as it finishes. It doesn't take long. It always comes out nice and crispy and I haven't had any issues with the fish I cook being greasy at all, but I also haven't used any of that store bought "pre mixed" batter or "fish crisp" kind of stuff. I make and season my own batter using the recipe I posted and the seasonings I want to use, same thing if I decide just to bread it instead of using a batter. I'll season the breading according to what I feel like at the time.