TMY2 is an entirely different beast, the grain structure is different and really optimized for smoothness. Note, however, that you can shoot the Rollei IR 'normally' and dr5 (slide) process it for very smooth results.
If you shoot the Rollei IR as IR film, just take care not to underexpose. If you do, the thin portions will appear quite grainy, particularly if you scan. This has been a problem for me from time to time when I use the deeper cutoff filters (#87 and 72) and the chance of underexposing is greater. So... I just bracket whenever I can and the most exposed neg is often the best. A well exposed neg, be it Rollei IR or superpan, can have quite smooth grain. Far smoother than HIE, in my experience. But not as smooth as tmy2. Closer to hp5+.
N.b. I shoot all of these in med format and have absolutely no issue with the grain I get from any of them.