I think the TDM 47s had/have is the wrong type now anyway, the DBSOs having been fitted up with the TDM used on DVTs to work with 86s/87s/90s. I personally think it is better to fit some 47s or 57s with the new type of TDM than to mess around re-fitting the DBSOs with other systems as that would give those locos the flexibility of also being able to operate with mark 3 and mark 4 DVTs. Personally I think about 9 47s or 57s should be fitted up with the right sort of TDM, life-extended and sent to Wales, with 6 of them on the 6 mark2 rakes with DBSO (one rake, inc. loco, would be spare so you have 5 mark2 rakes in service on any given day).
The other locos would be based at Swansea, along with Intercity 225s displaced from ECML by IEP, to operate Carmarthen and Pembroke Dock services post electrification by swaping with the class 91 on the 225 rake to operate the service into west Wales. Of course you need wires to Swansea to make it work, but stopping the wires at Cardiff is completly mad anyway.