Agreed, if you want more or less tension beyond what the standard die is able to do then you will need a precision ground mandrel (or precision ground bushings).
But you can customize neck tension with lee collet dies by running the die down a little at a time just until a bullet will not drop into the neck. Understand that the die is pressing the brass between metal fingers and a precision mandrel. If your brass is not concentric or evenly annealed the die will not correct a problem. And this is why you must ensure the brass is properly prepared to tweek out any gains in concentricity or neck tension.
Once you get to that point of just holding the bullet then adjust the die in a bit more (ie 1/10 of a turn) and you will have very little neck tension holding the bullet in the cartridge. Lower the die to add more tension to a point where you are comfortable it will not move under firing in a magazine (for a hunting rifle). Yes, this is not precise. But the practice is manageable and repeatable if you stick with the same cartridge base and mark your die/press to return it to the setting.
BTW...Just googled Norma and they make 7mm Rem Mag brass (I'll bet its pricey tho). Sizing 300wm lapua (if you can find it) to 7mm RM would be do-able but well into wildcatting territory and would certainly require neck turning. IMO, Start with concentric brass and you will get concentric ammo regardless of brand of dies or brass. Maintenence of neck tension will require annealing over the life time of the brass. I find this is something forgotten from time to time when you start feeling the seating process is inconsitent between cases. Hope this helps in some way...Lots to remember when creating perfection.