The improvement is stunning. The idea of Burson Opamp is very much like the HDAM of Marantz. Comparing the modded CD620 with Marantz SA15S2, the Burson Opamp, on the other hand, is more refine, more dynamic, and has better sound stage. It is surely worth trying considering the overall budget of CD620 and the mod parts is much less expensive than a new Marantz SA15S2 in that moment.
As stated by another C-hing below, TDA1541 can operate in NOS mode natively. There were some TDA1541 DIY modules with an embedded switch for NOS switching.