I'd agree there definitely would have been savings in that regard alright. However, if you have a look at flight prices around the last week of August from Belfast to Heathrow, the highest fare is 25 Stg. Most are either zero or 10/5 Stg.(Based on dates August 20th (out) August 29th (return).
I can guarantee you, those fares would never have been offered when the route was from SNN. Not even close and certainly not in August. There was no competition so they could charge relatively high fares and always did, and managed a decent load factor in the process.
Its a bit pointless to get into the SNN v's BFS thing(its done to death) but combine those fares from Belfast with 35k lower passenger numbers, and you can be sure the profit from those slots has to have been impacted...regardless of the savings made on handling
To say Aer Lingus had no competition at Shannon just isn't true. Ryanair operate services to Luton, Gatwick and Stansted which were undoubtedly in competition with EI. While EI aren't carrying as many passengers from Belfast at the moment when you consider the huge cost of self handling at Shannon for a relatively low number of flights, the fact that EI were being charged full landing charges at Shannon while they're getting reduced rates at Belfast, added to the fact that flight crew at Belfast are paid less than their counterparts at Shannon and are on inferior terms, I wouldn't be a bit surprised if BFS-LHR is making more money than SNN-LHR was making.