Yes, you need to put ample time in, with any detector, to learn it and know it .. both strengths and weaknesses, which they all have. I have the Simplex + as part of my Regular-Use Detector Team, and i run the 2.77 update. I wouldn't care for the 2.78 as the 2.77 seems to be the better performer in all modes.
I also keep the 5X9½ DD coil mounted full-time because I typically hunt in trashier, brushier, or more confined areas.
Almost always my Sensitivity is at the MAX setting, and if I have any EMI type issues I might drop it down one notch, or two if I really need to.
I have the Disc. set to reject only the 1st segment in the Field and Beach modes, and I usually don't use any Disc. rejection in Park 1. I seldom, if ever, use Park 2. A Disc. rejection of only the first segment will usually be just enough to reject most common Iron Nails, and in some ground mineral environments it also helps to eliminate some falsing due to challenging ground make-up.
Also, while I like a loud Volume for desired targets, I keep the Iron Audio Volume set at 1 or 2 for the lower loudness behavior which helps in ferrous littered places. Also, I always GB whenever I start hunting any site. I used all the different versions of the Teknetics T2 series since they first came out in 2006, most of the time using the 5" DD coil because I mainly grabbed it when hunting ghost towns and other places with ample Iron Nails and other ferrous debris.
I like the T2's with the '+' being my favorite. However, I had better performance from my Simplex + w/5X9½ DD in several old sites last summer and into the fall, and while it isn't a T2, an F75 or other higher-cost modern detector, they also are not a Simplex + which has some very well engineered-in behavior of its own. It isn't my main-use detector, but it does have its place in my Detector Outfit and with that mid-size search coil. Just give your unit some time, and if nails annoy you, just knock out the first Disc. segment and that ought to help. Also, I favor a slow-and-methodical sweep speed and not a brisk sweep.
Best of success to you and your Simplex +.
Monte