SOB is bad! first and foremost it's dangerous. placing anything thing hard of any nature on your spine gives too much potential for serious and possibly permanant injury if you imapct (fall). Ever notice linemen, cops, etc don't carry there? That's for good reason.
Holsters are very much an individual choice. What firts one isn't neccessarily gona fit another. Some of my guns feel better IWB, other OWB. Clothing also plays a parts as does the body of the person. (big, small, thin, not-so-thin) I've found the basic IWB is a very comon (i.e. favored) style. All depends on the afore mentioned conditions though.
I have carried at times a full size USP and use a OWB holster (Mitch Rosen 5JR). With heavier clothing this works for summer wear it wouldn't nearly as well. For those times a Rosen "Workman" is used to carry a SP101. I'll be ordering a new Rosen holster for my P2000sk and it will likely be an IWB.
You HK shouldn't need anything. It you can shoot it acceptably as it is, why chage a good thing?