@natem, I corrected the typo in the title of this discussion. @samidh recapped your question well. Your question is whether tamsulosin (Flomax) is still helpful even after a radical prostatectomy to remove the prostate. I found this research paper that partially answers the question:
- Impact of tamsulosin on urinary retention following early catheter removal after robot-assisted laparoscopic radical prostatectomy: a prospective randomized controlled trial https://pubmed.ncbi.nlm.nih.gov/23906190/

Have you asked your urologist this question? It looks like others would like to hear the answer too.