Good points Jeremy, thanks. I thought the ARF could run 500mW on 869.4MHz so my apologies for assigning it higher performance that it deserves.
When you say +18dB, I assume you mean +18dBm (since dB is a relative term). 18dBm is 63mW, 20dBm is 100mW. Even 27dBm, aka 500mW - the maximum legal power in the ISM band - is only going to offset 9dB of losses, which in the context of antenna gain/loss and, especially, in terms of shielding losses is not much, sadly. I don't know what the ARF receiver sensitivity is but I'd guess it's within a few dB of -107dBm, so not a lot of room for improvement there either.
I fear that any solution that does not include external antennas is always going to disappoint.