I’d say that usefulness of Collossus is a bit debatable – one: there are many perks that are more important for survivability (Battle Forged, Steel Brow, Underdog) or can give you more direct benefits to things affected by the HP pool (Hold Out, which negates injuries for the duration of the fight as well as helping with morale, or Fortified Mind that can make morale a trivial issue); two: it’s only really useful if you have a bro with a decent HP pool to begin with, as well as talent/good rolls on lvl-up. 25% of 40HP really isn’t worth a perk point.
So you need to carefully weigh different factors (base HP pool, fatigue pool, intended battlefield role, equipment used) before you can say with certainty that investing heavily into HP or picking Collossus is worthwhile instead of diverting your efforts elsewhere. Sometimes investing into more damage or accuracy can get you better results, especially when fighting in a tight formation.
That being said if your specific wish is to create a “whipping boy” of sorts that will stand on flanks surrounded by enemies, taking hits and being able to shrug most of them off, by all means compliment the other factors mentioned above with an increased HP pool – it will certainly not hurt. Just remember that there are some mechanics in place (such as gaining fatigue from being attacked) that can cripple your counterratack ability if you don’t factor them into your plans.