What kind of Muzzle velocity would I get??

We are getting 2890 out of a buddies 27" and yes it is pretty much a max load in his gun, but it's very accurate and single digit ES. I figure with two extra inches of barrel the OP might get a little more, but it will be a top end load.
 
A friends 338 with 29" 1-9 Bartlein managed 2850 (+/-) from a 300smk over 94.5gr Retumbo loaded to CIP length.

My TRG42 (26.5"??) gets 2675 from a 300smk over 90.5gr Retumbo at max magazine length.

Some barrels are faster than others, but I suspect 2900+ is going to be a tough number to reach from a 29-30" pipe with 300s magazine fed.

If you hang the bullet way out there and single load them, I suspect 2900+ will be a lot more likely.