Typically when weight matching pistons, you do exactly what you're saying you don't want to do ... that being, you weigh all the pistons and use the lowest weighted one as your goal. Since you don't state which exact engine you are rebuilding (you say BMW 6 cyl ... there were bunches of different BMW 6 cyl models), I can't look up the specifications to see what they are for your engine. However, 5.2 grams (0.18 oz) is very little weight. Most pistons have weight bosses on them which are designed so you can remove weight on the pistons to get them to where you want them weight wise. By removing the weight from these areas, you won't damage or weaken the piston in any way. And if done right, you can actually improve the overall structure of the piston so as to remove weak points (ie: stress risers).
To get done what you need to get done, I would suggest one of the following three things:
- Remove the extra weight from the other 5 pistons. Like I said, there is extra material built into pistons to allow this to happen. Because this is such little weight to remove, make sure you're doing it in small increments, weighing the piston you're working on often to ensure you're sneaking up on the number. Removing the weight can easily be done with cartridge rolls on a die grinder. Running it over a large area will remove minimal material from any given spot. Just to ensure you understand, this is done from inside the bottom of the piston. Don't remove skirt material length, only material from inside making sure not to bother machined areas.
- Get a stock replacement piston for the one which is out of spec. You could get a set of used pistons which are the same size and find one which is the weight you need.
- Buy a set of performance pistons. Most quality piston manufacturers will weight match a set of pistons from the factory. You buy the entire set and they will match, not only in weight, but in all other dimensions as well. I purchased a set of Federal Mogul pistons for an LS build. Not only were they weight matched, but the dimensions were within +/- 0.00025" ( 0.00635mm ) in the diameter as well.
Don't forget, when trying to balance your rotating assembly, there's a lot more to it than just your pistons. You also have to weight match the rods. This is done on both ends of the rods. When you get the rods weight matched, you need to balance the complete rotating assembly. This could involve adding or removing weight to the crankshaft and is based on it, plus the bob weight of all the piston/rod components, to include pistons, wrist pins, locks (if needed), rings, & bearings.
The other thing to remember when trying to increase your rpm is valve control. Yes, you don't want your bottom end flying apart due to being out of balance, if you don't have valve control, you'll introduce valve float which will absolutely kill power. To counteract this, you need to replace the valve springs with ones having a higher spring rate.
Everything above has to do with getting your engine to rev higher. Since you state you are rebuilding it, you'll want to ensure all tolerances from old parts are still good. Hopefully you're are fully measuring your pistons to ensure they aren't out of spec (ie: skirt wear, etc), and that the cylinder bores are still round and not tapered. There are many wear spots to check. Without a thorough amount of checking and knowledge of what to check for in the first place, I'd ensure you have a good source of information on those thing as you go through your rebuild process, or you'll just be throwing away a lot of time and money if you don't. Just a suggestion.
EDIT: Because there seems to be some confusion about what I've stated here, I'm including a video link about the subject about how/where to remove material in your piston: https://youtu.be/sPsC4BG80hE