ROM 3 machines have 1MB on the motherboard and You have an 8MB total limit I think. Pop 1MB off the expansion card and see if it works better.
Reading the technical manual for the IIgs my broad understanding of how the IIgs' RAM slot works is it's essentially laid out like a "RAM socket" instead of a bus socket, if that makes any sense. (IE, it's downstream of the RAM controller/memory mapper; it already handles generating the RAS/CAS signals in addition to a set of *multiplexed* address signals in which "address 0" is relative to the RAM chips making up the subsystem, not the IIgs' memory map.) Thus there should be no need to disable any RAM on an expansion card; the IIgs always puts the RAM after the built-in RAM, regardless of if it's a ROM1 or a ROM3. So it's more accurate to say the slot has an 8MB(*) limit, not the machine.
(* The TM notes that technically the slot only supports 4MB without the need of external RAM refresh circuitry, which the Sirus card must supply; this need is why Apple considers the "official" ceiling for the IIgs 4.25MB total in a ROM01, 5MB total RAM in a ROM03 in said TM. There are also ROM limits that come into play; the system won't use more than 8MB without a ROM patch, so an 8MB card always wastes some space. Technically the memory map of the machine could hold up to about 14MB, some emulators support this, but the physical slot in the real machines can't do it.)
Do you have another system you can test your RAM SIMMs in, or otherwise get your hands on some known good ones? The memory test is indicating it's getting bit errors when it's reading a word, and that points a finger squarely at either bad RAM or something gone badly wrong with some of the support circuitry on the card. (The IIgs only has an 8 bit wide memory bus, so there shouldn't be any way you could be misconfiguring the card that would cause bit-level errors; the SIMMs are themselves 8 bits wide. There's no interleaving or anything.) This *may* actually point a finger squarely at the refresh circuitry on the Sirus card being at fault. (Again, referencing the technical manual, a card up to 4MB has refresh signals fully supplied by the IIgs itself. Larger ones have to do it on their own.)
As a Hail Mary, depending on the condition of your IIgs, I might suggest cleaning the contacts on both the card and the edge connector. That *could* in principle cause spurious bit errors.
