Just saw this thread... and I WAS prepping for FCNC, HA!
Palma rifles shooting 155's are about the most insensitive rifle there is; meaning it doesn't matter much if your barrel is 24" or 34", your groups will be good. MV is higher with longer barrels, but when you get past 26"+, the returns are diminishing. You get a lot more fps going from 23" to 24" than you do going from 33" to 34". So getting a barrel of the nominal 30" length is fine, not much to gain by going longer in terms of MV.
In general, shorter fatter barrels are stiffer and can be more reliably precise in some cartridge/bullet combinations. But 155's in a .308, any contour you would use for a Palma rifle (light Palma or fatter) is heavy enough to be about as good as it can be.
The most practical consideration regarding the length of a Palma barrel is what Danny Biggs stated, which is related to sight radius. I discovered the past couple years that moving the front sight forward just a couple inches dramatically improved my sight picture at long range. This is the biggest concern IMO, not ballistic effects of the inches +/- 30".
If you want a long sight radius but don't want barrel, you can always counterbore ~2" at the end. John Whidden has done this on my barrels to prevent the front sight band from clamping the bore diameter down at the muzzle. Who knows how much that matters, but it doesn't hurt.
One important thing to keep in mind regarding Palma barrel length is standardization. If you aspire to be in a big team, there's always a question of sight radius, and the value of your 'clicks'. Ideally everyone on the team has the same sight and sight radius so they stay in sync with the coaches and other shooters on the value of wind. If you have an 'oddball' sight radius, it's a problem for the team to manage.
As you can see, most of my considerations for the length of a palma barrel are not ballistic, but rather sight related. In addition to shooting the bullet, the barrel is also a sight mount and you can't neglect that aspect of its function.
Take care,
-Bryan