I have a 21" 870...years ago when Remington loaded hevishot and it was fairly new to the market I bought a chep Undertaker hevishot tube for it..literally for a couple bucks at the end of the season...patterns are awesome...I could probably find better...but I'd go broke trying...
With 3" mag blend I am right around a 225 avg. in 10" @ 40yds.
One of the patterns....only thing I've done is polish the bore...
I'd try the cheap chokes like the Undertaker hevishot tube, jellyhead etc first....if they don't work well sell em and buy expensive chokes..
For a 21" barrel I just don't see mag blend patterning much better..
With the old remington loaded #6 hevishot (I think it was only 1.5oz) I put 187 pellets on a sheet of computer paper at 40yds...
Hevi13 #5s patterned well...Winchester flight control copper plated #4s pattern extremely well for lead #4s. I use them up calling fox at night and they do really well...