If all you do is replace the head pipe so you have no cat two HD dealers have now said no you do not need a tuner. If you change header pipes and air cleaner probably will.
First, you ALREADY have a high flow air cleaner on the bike (the Ventilator), so changing the air cleaner assembly is totally unnecessary unless you just don't like the looks of it. So that's the first thing your dealerships don't know their ass from a hole in the ground about. Secondly, whether you put an aftermarket header pipe on the bike, or simply remove the CAT from the stock header pipe, PRECISELY the same thing is occurring...you are flowing more air through the intake/exhaust. Many people on this forum have done just that...take the stock header pipe off, cut the pipe where the CAT is located, removed the CAT, welded the pipe back up, and reinstalled. But I'd be willing to bet that 99% of those that did that (or put an aftermarket header on the bike, like the Jackpot, Fullsac X Pipe, V&H "H" pipe, true duals, etc) have a tuning device on the bike to add more fuel and improve performance/drivability.
If you don't' put a tuner on the bike after removing the CAT, your bike will run, but how well it runs is a different matter entirely. Expect hotter engine temps, higher oil temps, and issues with throttle response. It's your bike, so do/don't do whatever you choose, but both dealerships you've spoken to are either ignorant, lying, or both. Or, they simply do not understand how their product operates. That would not be unusual at a HD dealership.
We all welcome new members here, and if you ask a question, you will generally get excellent advice because we've all been there and done that...usually multiple times. But if you choose to ignore solid advice by experienced HD owners, then that is certainly your prerogative. However, don't keep saying "my dealer said this, or my dealer said that" when it is a well established fact that many dealers don't know their ass from their elbow, particularly if you are talking to a salesperson. Your original (incorrect) statement has been more than adequately addressed.