Looking at the single threaded performance of the A10-5800K,it does look like Vishera will have much better single threaded performance than Bulldozer based FX CPUs. IPC of Trinity has already gone up by 15% when compared to Bulldozer which has L3 cache according to Toms Hardware. So I expect you will see around a 20% to 25% improvement in IPC overall with Vishera.
On top of this is likely to be clockspeed bump too,meaning that a 25% to 30% increase in overall performance is not far off for the same TDP and power consumption.
Also,due to the improvements in Turbo Core brought in with Bulldozer,clockspeeds in lightly threaded situations will be higher overall than either the Bulldozer based FX CPUs or the Phenom II X4 and X6 IMHO.
As a Core i3 2100 owner myself,I do like the look of the A10-5800K TBH. I suspect lightly threaded performance will be slightly lower but with slightly greater multi-threaded performance. OTH,since it can be overclocked,I might consider picking one up after I upgraded my main rig. However,I might just look at a 65W TDP A10-5700 instead for an SFF rig which can run some games. I can use one cooling fan and a DC-DC PSU! Hopefully,one of those Wesena cases would do the trick.
It will be nice to see how much people can overclock the CPU and IGP though- it looks like it will be fun!!