Don't Cardiff and Bristol control Filton's airways departures anyway?
Not all, and not always. Northbounds stay with Filton, as can eastbounds if Bristol have nothing to affect. Westbounds tend to stay with Filton until clear of other traffic and then transferred to Cardiff.
More airspace for Bristol couldn't come soon enough.
Bear in mind there are other airspace users. Much as the Commercial operators want more CAS around that area there's also a heavy training commitment by a lot of the Commercial schools (Oxford etc) to airfields in the area (EGTG/EGBJ mainly). Any CAS allocation will have to take the impact on that into account.
Of course - once it's all sorted it'll all change again when the new ICAO 3 airspace classifications take over
