The principle reason for increasing rod length is to reduce piston side loading at high outputs, it reduces friction which frees up some power, and helps increase the lifespan
of a racing engine. Within every engine there is only so much room, & a number of popular OE's are on the short side, the slanty isn't one of them. The long rod set-up on
the slanty does provide for a much superior piston package and height, providing higher compression and a shot at a squish-zone if one mods the head for such. The rod angularity piston speed/vs burn speed dynamic is one of the reasons some OE's have done what I was investigating doing to My next slanty build, offset bores to the
crank axis. Honda, Hyundai & others have offset the bore center from the crank axis as much as 12mm, this can do things in different ways depending on the goal of the
engineers, and since they can infinitely play with cam timing programming etc. the improvement in output and efficiency has gone up accordingly.
To the OP, if You really are looking to get some output, You're going to have to stall that thing at 3K & up anyway, but the build should based on what goals You really
want to max out at. If You don't have some competent headwork done with O/S valves & effective porting, You're not looking at a peak output RPM of over 5K even w/a
sizeable cam, it will have petered out easily by then. If that's the case, a stock/stockish head means You're going to have to make the most bottom end grunt You can,
long rods won't help at all here for sure. Bumping the squeeze to at least 9.5:1 by milling would be a better way of getting the job done, and cost less at the same time.