Res is massive, pipework to the res is long also, it takes quite a while for the heat to move round the loop. For 1C increase in res temp it took around 30 mins on load.

I was just thinking about this and my initial thoughts would be:

1 there is no need for a crossflow rad, you could use a standard rad mounted on edge horizontally.

2 mount a large res on top of the case with the rad right below it...this way cold water flows from the bottom of the res thru the rad before hitting a heat source.