Everything is on books out there but saying the MF is the same as the Bis.....Not arguments, just facts.
MF Engine R13-300 ( Thrust around 9000 lbf dry - 14.000 lbf AB )
Bis Engine R25-300 ( Thrust around 12.000 lbf dry - 15.500 lbf AB - 22.000 lbf in Max AB )
Bis upgrades: Aerodinamically cleaner fuselage spine, changes in fuel load for better perfomance, new SAU system, Lazur-Polyot equipment, RP-22M Radar, better ASP-PFD sight for increasing G tolerance while using the cannon, etc.
So , not the same plane, not the same FM, not the same flight perfomance at all.
If you want to do the job i can link here the real Mig-21 Bis manual with plenty of perfomance charts so you can take that reference information, climb on the Mig-21 inside DCS and make some flight test comparing the real charts with DCS numbers.
Then came back and share your tests here.
Then we can talk about numbers, charts, perfomances and FM.
Not feelings.
You can start with this, is quite famous around this forums, and the DCS Bis quite acurated to it.