Problems with high compression pistons and low octane gas are mostly confined to the USA. Since the 60's we have gone from 104 octane, 98, 94, and now 91-87.
My 1984 S3 from factory with a California specification, runs low compression pistons, fuel injection on 87 octane gas. The curve for this distributor has 17 degree static and tops out around 37 degrees at 4400 rpm. This is quite advanced as the combination of lean fuel injection ratios, low compression and low octane fuel produce a slow burning mixture, that requires the aggressive advance.
To prevent pre-ignition from well worn distributor, my shop times the engine at 3000 rpm, setting timing at 33 degrees and lettings the static fall where ever.
The new distributors have taken the low octane, lean burn, low compression pistons into account; hence have newer curves than the Jaguars of the 70s and 80s.
The XJ6 S1 curve on the graft would be a high compression piston with recommended fuel at 98 octane, using rich side carburation. This is a faster burning combination; hence the 3000 RPM timing is 30 degrees. What ever distributor you select, you would want to be near 3000rpm@30 degrees and set static timing as required to achieve that.
The Pertronix D171618 recommends 8 degree static for and S2, which would put you at 31 degrees @ 3000RPM ( The D171618 is not recommended for EFI so I pushed it to 10 degrees for my S3 analysis to achieve the 33 @ 3000)
Since you have access to a decent fuel grade the 9:1 pistons should not be a problem
Rgds
David

 

If you plan on going to 3 SU carbs, this is the manifold you want, or a one piece Mark X 3 carb intake manifold. The E-type triple intake manifold will not fit well in the engine bay.

https://www.ebay.com/itm/RARE-Jaguar...oAAOSw1g1fRBS0

https://www.ebay.com/itm/Jaguar-MK10...0AAOSw0Ztf7K3F

I have attached some notes on the cylinder head prep prior to block mounting: The jaguar manuals were written many years ago, so have gleaned notes from this forum as well as the Jag-Lovers forum as to current successful practices.

Rgds
David

 

Correction to Attached XJ6 Cylinder head Tips: Stud torque into block should be 27 Ft Lbs. NOT 20
Rgds
David