How much power will a stock 904 handle
It depends.
One has to determine what .15 seconds in the 1/4 is worth - what it takes to achieve it and what it actually costs in time and money. A lot of the "worth" depends on the rules you race under. The more restrictive the rules are, the more difficult it is to make "legal" changes for a .15 difference. Class racers, like Stock & Super Stock can spend thousands for less gains than that. Faster ETs are beneficial for qualifying, setting records and heads-up runs when like-classed cars match up during eliminations. i.e.:
B/SA 440-6 Challenger vs
B/SA 396 Camaro in Stock. In this transmission test, Chrysler did the R&D and saved racers that money.
As a car gets faster, the incremental gains will cost more. In other words, the next .15 gain in ET will likely cost more than the first .15 gain and so on. The faster you go, the steeper the increase in time & money. Eventually, hundredths of a second will be very expensive if at all possible to gain.
To help put things in perspective, a cheaper way to cut .15 in ET is the old rule-of-thumb estimate of 100 lbs = a tenth in ET. But can you remove 150 lbs legally under the rules? Bracket cars don't have these rules to go by but it may not be that cheap, easy or practical to do anyway even if the racer simply wants to go faster as a personal preference.
I swapped a set of unported 1.88" 587 heads for a set of unported 1.92" Indy LA-X heads. Not an exact apples-to-apples comparison because the valves were slightly bigger and compression was slightly more (about 4cc smaller chambers) but
everything else was the same. I gained at least 2 tenths, maybe 2 1/2. It wasn't cheap, but I was thrilled! I don't recall if jetting changes were required to re-optimize for the better airflow.
I gained at least .05 in ET with jet changes on a Weiand dual plane intake. I noticed plug coloring was leaner in the cylinders fed by the side with the deeper plenum. 2 # richer jets on the deeper plenum picked the car up. It cost me entry to a test & tune, race gas & all day of my time. A single plane Holley intake was still faster
on my particular combo with its preferred jetting. I didn't gain anything overall except the knowledge & satisfaction of knowing that the Holley I had been using was better.
Adding a 3" X-pipe & exhaust with straight through mufflers dumping at the rear axle picked my car up. I found over 60 lbs to remove from the car to compensate for the exhaust weight. After several races & testing to re-optimize the jetting, the car was between .05 and .10 faster.
I'm obviously one of those people who think that .15 in ET is a lot and often worth the time, effort & money. It's easy to make bigger gains by spending money and making major changes. Nitrous can spoil some people. Even then it can cost more to optimize jetting, timing, etc. because whenever you make a change, it can affect how well other parts work with the change. Some .15 ET steps can be cheap but will get more expensive. What are your rules, ambition, goals & budget? Multiple .15 increments can add up to sizable gains. Is .15 in ET a lot? It depends.