Hey, that looks like mine, I gotta go check my shop and see if it's missing... Only mine is 19 grams... but the Katana does have a 1/2" longer nose moment.
I've never hollowed under the engine, and since I usually end up adding a smidgen of nose weight anyways, I see no reason to. I'm certainly not hollowing that extra gram on my new 75 plane, I figure it needs a little cusion for the pushin there. LOL!
But I do sand my interior blocks smooth also thinking that it's stronger somehow. I know it applies to polished connecting rods, etc. Not sure if we're correct on balsa, but it does look nicer and makes it easier to see if your hollowing is consistent.
To keep the saw dust down when I do the rough hollowing, I use the exacto circular draw knife and pull out strips of balsa best I can until all that's left is the fine work for the dremel with one of those diamond ball bits that looks like it was dipped in magnetic filings. Then it's down to some sandpaper by hand to smooth out. How do you do it Sparky?
Keep on keepin on!
EricV