Ok so I have been doing some research on what aftermarket seats fit in the FD without modification. I have come up with some information that I think can be useful, and would like everyone with experience to post up what they have found. Pictures of the seats installed would be very beneficial so people can see clearance issues.
Seats that I have found should fit.
OEM RZ/Spirit R
Of course the OEM recaros will fit beautifully, and are by far the best fit for the FD.
Bride
Vios III
Artis III
Exas III
Gias
Zeta III (type S)
I know for a fact that the vios III fit and based off of those measurements I came to the conclusion that the other seats listed should fit as well.
Recaro
Profi SPG
Profi SPA
The Profi's may have slight rubbing issues due to shoulder bolsters. Again these conclusions are based on dimensions only not personal experience. Some people seem to have no issues.
Pole Position
Memoryfab
S58
S68
S98 (reclinable)
Corbeau
CR1
A4
*Both corbeaus seem to have some minor rubbing issues
It seems that around 580mm is the upper limit for the width of the shoulder bolsters, and I'm not entirely sure how wide a base will fit, I would guess around 470mm is a good width.
These are the main ones I have found so far. Please post any info you have on seat fitment, so we can get a good seat guide going.