Like others have pointed out, oversize when using cast is a good thing. My 1894 slugged at 0.430" and I use 0.432" diameter cast bullets with good success. I have settled on an NEI #252 240 grain SWC-GC, and with H-110/W-296 or Li'l Gun, I can push them to 1,740 to 1,750 fps with good accuracy. For critters, that is about my favorite load. I haven't messed with any bullets lighter than 240 grains in my 1894, and bullets heavier than 240 grains didn't give me quite as much accuracy (in mine). Maybe I just didn't find the sweet spot with those heavier bullets, but I don't think there is much the heavier bullets will do that the 240's won't do just as well.
If it turns out that I need a heavier bullet than a good, cast 240 gr with a muzzle velocity of 1,740 or 1,750 fps for the sake of better penetration, then I probably brought the wrong rifle.
The only micro-groove barrel I have is on a 336C in .35 Remington, and it shoots cast bullets just as well as my ballard rifled barrels.