My foggy memory reveals a little more ...
IBM's license to the 486 processor at the time did not let them sell the processors as stand-alone chips. They had to be packaged as part of a motherboard or a complete system. So the only way to get them to end users was to sell upgrade packages or complete motherboards.
So yes, they probably were used as upgrades to existing IBM PS/2 machines. But it would have been a processor card or motherboard swap, not an 'overdrive' type upgrade.
Yes, indeed. One use of the IBM Blue Lightning processor (also known as the IBM 486SLC) was in Evergreen Technologies' "REV TO 486" module. That module was designed to upgrade Intel 386SX systems to the IBM 486SLC processor, and also included a 16KB L1 cache. The module would double the clock speed of 25 MHz and 33 MHz systems, and triple the clock speed of 16 MHz, 20 MHz, and 25 MHz systems.
I successfully installed such a module in December 1995 on my IBM PS/2 55SX and the upgrade worked fine. It upgraded the 386SX/16 to a 486SLC/48, allowing the Windows 3.1 system to run quite a bit faster.
Processors like the IBM 486SLC were more than just a product of reverse engineering. Intel allowed IBM to improve it's 386 and 486 chips for use in IBM computers. From http://query.nytimes.com/gst/fullpage.html?res=9E0CEFDE143DF936A25752C1A964958260&pagewanted=all: "Intel and I.B.M. have a technology-sharing agreement that allows I.B.M. to produce its own versions of the chip, and the Thinkpad 700C uses a variation of the i486SL chip, called the 486SLC." See also http://en.wikipedia.org/wiki/IBM_486SLC.
...Within moments of the news last week [the article is 15 November 1992] that the Intel Corporation was introducing a new chip designed specifically for portable computers, Compaq, NEC and AST Research announced new notebook computers that would use it.
The new chip is the i486SL, the fourth member of Intel's 486 family...