My advise, is do nothing now!
Like most things today, the priority is never getting detonation because of things like hard machined edges in the combustion chamber, and never going too high on compression. Pre Mid 1980 to October 1985 4.1's had compressions in the 9.35:1 range and needed hi test (our 95-96 RON unleaded) to run correctly. The Aussie market never got leaded fuel until late 1985, before that the octane was well over 97. Later (post October 1985) 8.8:1 compression carby 4.1's could feed on the state mandated 91 unleaded. I think your 87 gas is our 91 'petrol', and your 93 gas is our 95 or 96 grade because some boffin rates pump octane by a different measure here.
There's nothing bar a good 3 or 4 angle cut of the seats, and a very light debur of the edge of the combustion chamber with fine sand paper before the head is cleaned and assembled. The flow of this head is just wonderful, and because its alloy, even the so-called "worst" casting is going to handle a great deal of compression before detonation sets in.
Some info for you on my searches. Check with Aussie7Mains, JD, or MustangSix about the details. Depending on the casting, you can do a few things. If its a late model casting (HF-6 or later) like the one without the EFI rail that is in Jacks photo below, I'd do nothing to it. It has set of big valves, great swirl properties, and the best valve inserts money can buy. Any modification will most likely create problems with detonation if you try to go over 8.8:1 compression. The fuel consumption, emissions and horsepower levels are perfect for a street engine. With the stock lift cam, there is any easy 185 hp hideing under its odd shaped rocker cover. These heads are much like the 86 EFI Mustang 5.0 heads. Great for a stock engine, but a liability if you are going ballistic with the compression ratio.
With the earlier heads (HF-5 and earlier), the intake valve is a about 50 thou smaller, but the heads chambers are unshrouded and flow as good as the HF-6's. If the 1.8" intake is added to the head in place of the 1.75" valve, you get better power, and even less detonation prone. You may be able to go a little more than 8.8:1. But all this discussion is relative to the South Pacifics old leaded gas that these cars ran. It had 97 RON/MON octane, and the 4.1 the HF-5 head came from was pegged at 9.35:1. On a 4.1, a head shave of 25 thou will raise the stock 9.35:1 up a little to 9.7:1, no problem with 93 RON.
If you go to the 93 octane, then 9.7:1 is possible.
What you get with a cross-flow, especially the later 1986 on HF-7's, is bag-loads of torque, I'd say up over 25% from the earlier log headed 250 engines, and perhaps 5-10% up on a 2V head 250. From 600 rpm to the 4500 rpm readline. If you have a 4.1 x-flow, it will run like a 5.0 2-bbl engine around town when you lugging it. If you give it a spin, there will be an easy 130 hp with a stock ADM 34 Weber on a 4.1, still using a similar cam profile to the US 200. The 4.1 x-flow was only a 126 or 131 hp engine, unmodified, without EFI, and between 149 and 161, then 162 hp. But with a 280 degree cam, and the right carby (500 cfm 2-bbl) you can get 195 on a late unleaded low compression engine with no other modifications. Dick Johnston was going to release an HO carby engine with the compression rasied of Shell Optimax 95 (your 93).
The headflow isn't the issue, its the cam, timing gear, the quality of the balancer, the size of the carb, and at about 290 hp, the rods and valve gear which are the limiting factors. Turbos could go to 350 hp with no bottom end modifications.
