If you want to know the opinion of someone who has played with Slackware, in different x86 architectures.
The biggest improvement will be observed by switching from i486 to i586 with the use of MMX extensions, especially in older systems. I.e. MPlayer and Xine will run better in a old machine.
However, by switching to i686, will lost compatibility with some processors, will not see substantial improvements beyond i586 and will open a cup of worms. Various applications will crash or fail mysteriously. I guess because of (wrong) optimizations applied by compiler.
In fact, if you're curious how it will look on a i686 Slackware, try Arch Linux. You will notice some instability.
Therefore no major distributions use i686 architecture as default, the common case is i586.
If Patrick wants to change its Slackware architecture, I suggest him to use the i586 too, if he wants to avoid stability problems.