Hi and welcome to the forums!
The GT Performance Package for the 2020 model year retails for $5195 on the Ford site. Did you ask the dealership about the price difference?
As far as raising prices, I only see dealer markups on models that garner competitive offers from buyers such as limited releases. I've never come across a dealer that offsets rebates or incentives by raising the base price of price for options.
As far as options you want . . . I'm the wrong guy to ask that. I purchase vehicles about every 3 years and often find the features sold in base models are very nice and an upgrade from previous years. I avoid GPS, Lane Departure features and I'm not into mega-watt car stereo systems or leather seating.
2012 Mustang GT, Roush 2300 Black
2006 Mustang V6 Manual Legend Lime Sold
2019 Charger Scat Pack White
2016 Jeep Willys Silver Vin 060 GBNF
2016 Jeep Willys Silver Vin 059 Sold