These standards are a rat’s nest, unfortunately complicated by marketing and unwitting vendors.
The SFF-8087 (and 8088) is a standard for exclusively the mechanical connector shell/plug. The actual electrical connection specification accompanying the 8087/8088 connector is the SFF-8086 standard, which is a 4-lane solution rated at 10Gbps per lane. It is rarely used for SAS3 applications for this reason - negotiation will work, but throughput will be limited to the maximum the connector is electrically capable of. An 8087 SAS3 card exists for the purposes of maximizing profits.
There is a similar conflation with the SFF-8482 standard - SFF-8482 is for the mechanical structure of the dual 7-pin + 15-pin connector set. It defines the physical structure and mating, pin offsets for contact sequencing, means of retention, etc. It has been paired with several electrical specifications, particularly SFF-8678 (originally SAS1, later updated to SAS2), SFF-8680 (originally SAS2, later updated to SAS3 - this is why 6Gbps is not mentioned in the SFF-8482 document when they mention the specifications used in conjunction), and SFF-8681 (SAS4). The actual cable names should be according to these electrical specifications which also refer back to 8482 and thus encompass the whole connector assembly, but they are not. Most SFF-8482 cables you see for sale are likely SFF-8680.
SAS1 = 3Gbps
SAS2 = 6Gbps
SAS3 = 12Gbps
SAS4 = 24Gbps
SFF-8086 (+ 8087/8088) is primarily used as a SAS2 connector, but is capable of up to 10Gbps per lane. Will negotiate at 12Gbps, but top rated speed is 10Gbps.
SFF-8482 is the framework used on all 15+7+7-pin connectors (and even U.2 connectors) - SFF-8678, SFF-8680, SFF-8681. Each of these three builds on the former.
SFF-8678 = 3Gbps or 6Gbps
SFF-8680 = 3Gbps or 6Gbps or 12Gbps
SFF-8681 = 3Gbps or 6Gbps or 12Gbps or 24Gbps
All of this information is based exclusively on the standard documentation available from SNIA, ECIA, IEEE, etc. All of the above is spread across the SNIA documents for those standards explicitly mentioned, and maybe a few others that are more general (SFF-8024 perhaps) - the other sources may be useful for additional context on design choices. Many other things you have read on this topic are liable to be wrong, and are probably based on marketing material from vendors - anything that does not explicitly reference the actual standards is probably based on inaccurate sources (I’m looking at you, Wikipedia).