I could have titled this a number of things, but the gist is that we're beginning a new run and the DM has allowed both Spheres books and Path of War. As I'm a fan of both I was considering rolling up a Fiendbound Marauder Warder (Gives up some defense buffs for a reach grappling claw and curse based stuff) and was looking into using the optional rules of spheres of might to access it without multiclassing.
The option is that you can give up either half your feat progression for Proficient sphere access (trading 5 feats for 10 combat talents) or all of your feats for Adept (trading 10 for 15), with Warder giving me 4 bonus feats meaning I lose technically only 1 feat if I go for proficient. Using these I would go into the Wrestling Sphere and access to the Battered condition for better combat maneuvers while using my PoW maneuvers as more enablers to get the grapples going and to make up for damage because I plan to not make too many full attacks. Is this an idea worth investing in or should I multiclass to access the spheres? Or is this just being too complicated and simply stick to Warder normally?
Would taking the Extra Combat Talent feat make things easier for you?
Yes and no. That is the easiest way to do it but actually getting the progression gives you access to a Martial Tradition (thus several spheres for free anyway) and if I did my math correctly getting the progression nets me more than just taking Extra Combat Talent. Sacrificing the 5 feats makes me lose a net of 1 feat from bonus feats due to warder levels, and gains me 10 talents with Proficient access + whatever I get from the Tradition.
I had given it some thought before about the best way to combine the two. My best GM instinct told me not to. Using both in tandem you can get some very powerful and likely unintended options.
I wouldn't combine the two at all quite frankly.
I guess it depends at least partially on what you're trying to get out of it. Both Spheres of Might and Path of War are largely patches to the same problem - a lack of first party options for martials - and do a pretty good job of it in isolation.
In your case it seems like you're trading out one kind of combat effectiveness for another, and I'm not sure that the character concept you're sketching out needs it to be mechanically effective or to work flavorwise, so it seems like the only way to justify the extra complexity load is if you're going for major cheese. That's not necessarily bad, but it depends on (1) the rest of your group's composition and playstyle and (2) what feats you're trading out for what talents. You're the expert on (1) and I'm not really smart enough to speak to (2).
OTOH SoM does address default martials where they're weakest - out of combat - so it might make sense to take a PoW class and trade out some feats there where they help fill out your concept. Maybe your invisible demon arms can burrow through the earth or have creepy eyes that are always watching. Swapping out your default proficiencies for a martial tradition can also have a potential flavor payoff.
Keep in mind that many Spheres and talents count as feats or duplicate their effects to some extent.
Take Barrage Sphere for example. The base Sphere itself counts as Point-Blank shot and Rapid shot, so you count as having those when it comes to other feats.
I'd say in the long run swapping feats for Spheres is probably the better deal.
and just for emphasis, quote from the book on Associated feats.
Associated Feat: Some spheres and talents overlap the
function of existing feats. Such a feat is listed in the talent as an
associated feat. Talents with associated feats allow a character
to qualify for feats that have the associated feat as a prerequisite,
including any prerequisites the associated feat normally requires,
and for abilities that modify the feat’s function (such as
a mythic version of the feat). Unless noted, talents do not stack
with their associated feats. Any time you would gain an associated
feat, you may instead choose to gain the sphere or talent
it is associated with. You must still meet the prerequisites for a
talent gained this way, such as possessing the base sphere.
Spheres are not as generally applicable as feats, since they mostly work with the standard attack-centric aspect of SoM. That means that it's not worth it as a trade unless if you have some specific build in mind that utilizes SoM in some way.
There are spheres that work wonderfully with Path of War, though. Spheres like Alchemy, Gladiator, Scoundrel, and such do not really care about what action you use and thus can work well with maneuvers.