I have no experience with your sander; but have the RO 150. (I have posted something like this before) When i got the sander, i did not know anything about varying stifness of pads. Mine, as it turned out, had the softest pad. I had been using it mostly for sanding hardwoods with no problems. If sanding softer woods, I still had no problems.
Eventually, i found myself sanding down a rough surfaced lazy susan. The center piece was a 3" square of black locust with a very solid hard knot taking up most of the center of the square. Around the knotty locust were two pieces of red oak and two pieces of white pine arranged in opposing corners. I was having trouble getting the three different woods (from extreme hard to very soft) to smooth out evenly. finally, i noticed I was making some serious cupping in the soft pine. I do not have a belt sander (to many bad experiences with my old one), or a drum sander, so I sent out my SOS to the FOG. Some kind soul (as all FOGGERS seem to be) informed me that I was probably using the soft pad and i should try the hard pad. An hour and a half round trip to my local toy store and mounting the new hard pad to my RO 150, and a few minutes to level out the rough spots and a lot more time to bring the hard woods down to level with the soft woods and I have never gone back to the soft pad. I do have a couple of projects in mind for this upcoming winter season where I am quite sure the old soft pad will come in handy. The amount of concave curve I will be trying, I am sure will be sandable with the RO 150.
If to tight a depression for my sander, I will keep checking back to find out how you managed your project. I am stuck on this slippery slope and continue to find ways to get sucked down deeper and deeperanddeeperanddeeper.
Tinker