It has little to do with power level other than with high power usually you also have high rpm. It comes down to rpm/seat time vs valve/valve seat material. Fuel definitely plays a part, as does tuneup for that matter. They both dictate how much heat you are dealing with in the first place. Valve materials such as SS suck at heat transfer and why they are the first thing to go when power/heat/boost go up and the valve can't cool itself which leads to tuliping the valve head. Inconel is much stronger at high temperature but is not a lot better than SS on heat transfer. Their strength is what keeps them alive at extreme temps and at higher rpm or larger cam lobes the copper alloy seats help keep the temp down and parts happy. Now based on a max effort combo there are several factors that may lead to using a titanium valve. The biggest reason is heat transfer rate, not the common misconception of its used because it's lighter. This is a important feature but it's ability to manage heat in the valve is its true worth and at the same time being strong enough to handle all the stress it is subjected to. It's light weight obviously allows higher engine speeds with lighter valve spring pressures which does help some combos to rev higher more easily than a heavy valve accompanied by a heavy spring. Some of the new high tech small springs help this situation if they have the pressure required to allow the valve to remain in control and rev higher. This is mainly due to the fact that about 30% of a springs weight counts towards the overall valve weight just as the retainers, etc count. So a 1.550" spring with the same specs as a 1.650" spring on the same valve etc will rev higher before going into valve toss. The Ti option does come at a price though, they are expensive as hell and similar to an aluminum rod, they become a consumable and need to be replaced at regular intervals. There are also some pretty major advances in seat materials, several of which are way safer than copper beryllium which can kill you. Moldstar has some great alloys that actually change on a molecular level when subjected to heat and impacting it forms a boundary layer that is so durable the seats pretty much don't wear. These are used a lot in NASCAR and along with hard coated valves the valve job stops being a concern. I have seen valves that came out of these engines that have run qualifing and a 500mi race then sold as used parts. You can not tell they have ever been out of the box and the seats looked virtually new as well. Hope this helps and sorry to write such a novel.
