Move 15ft. using a 5ft. step. A combination of Following step and Outslug Sprint.

[*Khan*]

Hi
Following step allows you to move up to 10 ft. as a 5 ft. step using an immediate action. The triggering action is an adjacent enemy who are making a 5 ft. step.
Outslug sprint increases your 5 ft. steps by 5 ft. when using outslug style.
Will this combination let you move 10 ft. on your own turn and full attack AND move 15ft. on an enemy turn if the enemy 5ft. steps ?
(you still have to follow the enemy and end up adjacent - but it could be in a flanking position)
Or will the text in Following step “MOVE UP TO 10ft.” be the limit?

Following Step.
Outslug Sprint.

[Cuup]

You're movement with the Following Step feat is NOT a five-foot step, so Outslug would not apply. It specifically states that "you may move up to 10 feet". Even though you're specifically making a 5-foot step with the Step Up feat, that wording is completely absent from Following Step.

[That Crazy Alchemist]

I've always hated the wording of Following Step. It directly calls Step up, then proceededs to spend the entirety of a paragraph explaining how it isn't like Step Up in the slightest. They would have actually saved more ink saying "When an enemy takes a 5 foot steps away from you, you may spend an immediate action to move 10 feet. This movement does not provoke attacks of opportunity, and you must end the move adjacent to the enemy".
Boom, slightly fewer words, and far less confusion.

[Azothath]

so that's Step Up then Following Step(which supersedes Step Up) for two feats to follow one enemy up to 10ft on their move. Hmmm... sounds like a bunch of feats just to maintain threat on an opponent.

Outslug Sprint does combine with Step Up as a "5-ft step" is a defined free move option and the key word is used in both feats. HOWEVER, Following Step is 10ft and gives you back your "5-ft step" for the next round, so I'd have to say no stacking with this feat. It does seem plausible so your home game GM should give it to you at the cost of using it on your next round.

[*Khan*]

The text on following step is this:

Benefit: When using the Step Up feat to follow an adjacent foe, you may move up to 10 feet. You may still take a 5-foot step during your next turn, and any movement you make using this feat does not subtract any distance from your movement during your next turn.

Normal: You can only take a 5-foot step to follow an opponent using Step Up.

I still read it as you still use the step up feat (an immediate action), but you can now move up to 10ft with it and it does not influence your movement on your next turn.