Really?, are you saying Mode S will initiate a TCAS advisory?

I believe TCAS is based on ADS-B but requires a certified GPS input or the information is ignored by a receiving TCAS equipped aircraft, don't know why this is when was the last time your GPS was totally wrong, I think better a couple of false triggers than bumping into something, again I think the SE2 has a certified GPS so would trigger TCAS. Feeding a TRIG transponder with the GPS from a PAW, which is uncertified will not trigger TCAS. On a PAW a FLAM unit can be directly connected to it, so no ground support needed just more costs. Don't know if Flarm traffic will see you?
Other catch feeding a PAW into a transponder for a certified aircraft, the installation must be carried out by a certified engineer and signed off by them. On a permit aircraft can be fitted by you but the LAA do the signoff. Both need a flight check. So thinking about it a PAW with a mixture of ground support will tell of most things around you but with no transponder only other PAW equipped traffic will see you. With an SE2 only sees ADS-B traffic plus Flarm again I don't think it tells FLAM only traffic you are there, buts costs a little extra a year, but does tells all ADS-B traffic your position.
Now if PAW added ADS-B with a certified GPS that would be something. I think there so much computing power in a PAW they could add TCAS but the certification would be very expensive.

Yes - 37 years and 25,000 hrs flying Boeing 747 etc. TCAS was developed before ADSB was even thought of hence it uses transponders Mode A C or S.
There are no - repeat no - transport jets that only use ADSB in...........

Sorry totally wrong.

The confusion is based on misunderstanding how all this airborne equipment works. The various jets I flew were gradually fitted with ADSB-out in response to ATC improvements under FANS. Areas such as the Pacific and Northern Canada now use ADSB signals for ATC as there is no possibilty of conventional radar in such remote locations. This was in addition to the already fitted TCAS and transponders and makes no difference to that.
The next development is ADSB-in to allow the display of target labels of proximate traffic with a view to "climb in trail" etc ATC functions. Non of the 777 and 747 aircraft in the airline were fitted with this and still aren't.
Some airliners do have it but not as a standalone replacement of existing TCAS/transponders thus all light aircraft with any transponder will be seen and acted upon.

The SIL=0 is of purely academic interest unless you are using a simple ADSB transmitter with no transponder and getting close to the big jets.........

From Wiki:-
'TCAS and its variants are only able to interact with aircraft that have a correctly operating mode C or mode S transponder. A unique 24-bit identifier is assigned to each aircraft that has a mode S transponder.'

What if any, advantage would I get if I added a Trig TN70 to my Trig TN21 Mode S transponder, taking advantage of CAA grant?
I'm considering doing something, but not sure what.

You would be maximising your electronic visibility to all other airspace users.

Adding a TN72 would give you ADSB to SIL=1 so that eventually ATC will be able to see you by ADSB. Currently though in the UK there is only Aberdeen ATC that can interogate ADSB to SIL=1. Not a lot of advantage at the moment really outputting to SIL=1 until ADSB is adopted by all ATC units as far as I can see.
If you are currently ouputting ADSB by ES Transponder connected to an uncertified GPS you are transmitting to SIL=0 which is fine for other GA aircraft to see you.
SE2 incidently outputs ADSB to SIL=1 in the UK