I actually wonder if a semi-centralized Mercian England would fare better against the great Heathen Army. Before the GHA, the vikings were just raiding. There was little incentive to really put in place the countermeasures that could have halted the GHA. So if anything, a Mercian England would have just centralized power into a single place. A place where the GHA could focus on attacking. IOTL the GHA was operating for thirteen years from their landing in 865 till their defeat at the Battle of Edington in 878. During that time they were marching all over to subdue Mercia, Northumbria, East Anglia, and Wessex. If Mercia semi-centralized England, then they can just head straight to Mercia. If they prove victorious, they can use the very system the Mercians put into place. If it is still a decentralized system of tributary systems, than it is likely Mercia has weakened the others already and these other kingdoms probably wouldn't overly protest much if the GHA came in and wrecked Mercia.
So while a Mercian England would probably be able to bring up more resistance against the GHA right away, if it fell it would basically allow the GHA to take it all by cutting off the head and taking its place. Mercia also wouldn't have the time to enact the sort of systematic changes against the vikings Alfred the Great was able to during the time of the GHA and during the subsequent Danelaw.
If Mercia survives and united England though, I imagine an earlier focus would be placed against the Welsh. Mercia did border them, and they were clearly a problem based on Offa's Dyk being constructed. A more northern base of power would also make it more like that England would look to taking the Scottish Lowlands before Scotland is really united.