I'm up for some noddy maths:
Ka power: ~60BHp -> ~50BHp @ wheels Ka weight: ~1000kg (with dervy + half tank of fuel)
effective Ka power to weight: ~ 50BHp/ton
Volvo power: 177BHp -> ~160BHp@wheels Volvo weight: 1530kg (with Tim +half tank...)
effective volvo power:weight ~105BHp/ton
Ka + volvo weight (incl Tim + dervy +.....): 2530kg
-> effective p:w of volvo + ka ~ 63.2 BHp/ton
Given the low-rev, high torque delivery of the volvo engine, plus a reduced effecive drag coefficient, I'd imagine the volvo towing the Ka would be quite a bit quicker than the Ka on it's own.