Thanks Peter - I agree with your and Tinus' point on the difficulty with getting torque gains low down in the rev range simply by improving the air characteristics.
It appears from my research (I have a lot time on my hands

) that the supercharger option version is the most effective from obtaining torque evenly through the rev range. I have attached a dyno stat from one of the companies that apparently was from a real life car they did. +-R125k quote to do that. (I'm sure it can be done for about 80k but I don't have the data to back this assumption up.)
Given the varying views on what could be done and the gains achievable/not I'll try and measure the modifications as objectively and independently as possible so that, if nothing else, the next guy that wants to do this will have yet another data point to work from.

My suggested process is as follows - please add to it:
1. Test the car as current on a dyno that isn't the dyno of the company doing the work. When testing record air temp, time, tyre pressures, car setup (weight)
2. Record on a GPS a 0-100km/h run on a straight flat road, three runs. Record same data as above.
3. Record on a GPS a 0-80km/h run on a straight flat road, three runs, this time with my Caravan in tow. Record same data as above
4. I have been recording fuel consumption in varying conditions for years now so i have a good data on that.
(this is the standard car - original air filter, exhaust, nothing ever done to the engine it, except serviced by Gtech 5k km ago.)
5. Take car to have a 63mm stainless steel free flow exhaust fitted + steel branches (ceramic coated) + UniChipQ4 - hard wired in, one map only, with a bridge plug
- they will do their own Dyno test before and after.
6. As there are no K&N air filters in the country at the moment (arriving mid July according to K&N.) I will replace this myself and then repeat steps 1,2,3.
7. I am going to Mozambique in August for three weeks so will have good fuel consumption data (point 4) as I'm towing again.
This is not a massively expensive exercise but the data would be fun to know, for acceptable financial risk (given that i probably could just drive more aggressively with the revs - per Peters comment.
My conclusion on the filter is that I will give it a try (for 15 years I used washable oil filter on my motorbikes (BMC) - lots of dirt), including a planned 5 week to Botswana in Dec. I will take a replacement OEM paper air filter.
Happy to share the quotes I've received and who from but would prefer on a PM out of respect to those companies.