A steel guide rod would be allowed as it's internal. What would disallow it is the following clause:
USPSA 14th Editition Rule Book - Pg. 91
Appendix E US Divisions
Maximum weight, Yes, 2 oz. over factory.
I think, but am not sure, the answer to this was published in Front Sight when somebody asked about replacing the plastic guide rod in a Glock with a steel part.
Basepad bumpers would be legal if they are used on the stock mags included with the gun. If not, they aren't, even if they are Beretta parts.