when you say 300m is that the slab area ? Shame your pipe spacing is for old gas boiler, 100mm is much better.
id agree on being oversized, rule of thumb is 30w per m2 on new regs.
My aquera when we first ran it in march which was a cold snap, below zero. We had no loft insulation and all the downlighter holes were open upstairs and was getting a cop average of 8.6. flow temp of 25 and this was too hot.