Just a bit of 'blue sky' thinking here, but it's always seemed odd to me that on an electrified line, the only fast service between Southampton and Portsmouth is a grotty and often crowded three-car DMU.
Wouldn't it be an idea to truncate the Cardiff-Portsmouth service to Cardiff-Southampton, and use the fast path freed up by that for the Weymouth-Portsmouth service...?
There are two things being added, one of which is an additional fast Portsmouth to Southampton service. The other is an extension through to Weymouth, but we are only making an assumption (albeit fairly logical) that it will be the stopper that gets extended to Weymouth, it could go either way, but I suspect we won't know for sure until they publish something.
At the same time, GWR are already intending to replace their grotty 158s with longer trains (5 car Turbos maybe) and that may also happen around 2018 anyway?
My personal view is that a fast "SW" path via Netley will be available, as long as it runs ahead of the existing flight of three fast trains operated by SN ex Victoria, GWR and SN ex Brighton.
I don't think they will therefore need to cut back the GWR service. A fast "SW" running maybe 15 mins ahead of it from Portsmouth ought to make space on the "GWR" units for people who really do want to go to Bristol and Cardiff etc.
What no-one has mentioned so far is if there might be problems squeezing in the odd GW Brighton service that runs a couple of times per day westbound (and once eastbound) in addition to the normal 4 trains on the Netley line...