Input display is probably the most constructive thing to improving input. If you have problems with Mature rekkas I'm betting you'll run into the same problems as I did doing Kensou's rekkas under pressure: His QCF's turned into rekka [DC] DP gut punch. Or Ralph, his Command normal into qcf+c turned into command normal super. The reason for this is that there are tiny tiny input errors. For example my Kensou problem was a result of QCF's being rushed as well as the second punch coming out a taaaaad early so the punch would land on the down forward rather than the clean forward. For Ralph it was that when going into down forward, I would sometimes hook too hard with my ring finger and it would turn into d, df which counted towards the super's buffer.
Input leniency has breeded a generation of fighters with bad habits. Mashing DP's or inputting your DP as f, df, d, df, f, or a similar derivative will be accepted as a DP to the game due to input leniency but will also build inputs towards future buffers. Learn to do everything perfectly clean and you'll be ok.