Godfryness, if that's what you want than go for it... I converted my early Sig-556 rifle into a close copy of a Sig-551 with an original 17" Sig barrel w/ grenade ring, stamped steel lower receiver and fixed sights. After everything, I'm very happy with it. The only thing I have left to do is to find somebody to "fill and reshape" the front-end of the Sig-556 upper receiver to mimic a true Swiss made Sig-551 receiver and re-engrave it with "SIG 551." At this point, I'm at the $3,800/$4,000 mark. In my situation, its easier on my wallet to buy a few parts here and there since every time I get a few thousand dollars saved, Murphy's law kicks in and those funds are rerouted to the trouble spot. So, buying a real Sig-551-2 wouldn't work in my situation since I'm an average Joe on the verge of retiring from my average paying job... Would I like to have a real Sig-551-2... Hell YES!!
Yes, my Sig-556 conversion isn't a true Sig-551 rifle, but neither are all those MKE, SW, POF, etc... "MP5" clones are not real "MP5" or "HK-94" carbines, just "MP5" clones... That includes those HK SL8 converted into a G-36 clone. As for the re-sale value issue, this rifle falls into the same category as my real HK-91, HK-93 and HK-94... I will never sell it
With that said "I" don't see anything wrong with converting something into what you really want. BTW, I saw a SBRed POF-5 "MP5" clone last week and it looked pretty freaking cool!!... I was going to SBR my HK-94, but now, I think I'll go with a new POF-5 pistol and SBR it into a "MP5" clone.