I would at least change the design to reduce dimensions G and I from 0.3398" to 0.3385". No sense allowing the excess gas to excape around the bullet as it travels thru the "LEAD".
If you're going to outside neck turn the necks of your brass casings, you may want to reduce the dimension H from 0.372 to 0.370".
And as yorke-1 stated, determine your preferred cartidge overall length (COAL) with the bullet you intend to shoot, and then match the "LEAD" length to be compatible with your preferred COAL. Your "LEAD" dimension is shown as 0.130". I went with 0.208" "LEAD"
on my reamer design, intending to load the Berger 300gr OTM to a COAL of ~3.945".
I see. I didn't realize the reamer was already purchased. I don't know if it's possible to have the reamer reground to smaller dimension or not, and I don't know what sort of fee would be charged. But with respect to the "LEAD", yes the only alternative I can think of is the use of a .338 neck/throat reamer to lengthen the "LEAD". That steel has already been removed from your reamer, and there's no way to add it back to lengthen your "LEAD" with that reamer.
I have heard of re-sharpening services that aren't all that expensive from reamer manufacturers, but I don't know what might be possible to alter your reamer dimensions before you cut your barrel chamber with it. Now's the time to do something like that rather than rushing into a chamber job with a reamer that's lacking in any significant way. It only gets more and more expensive once you've cut and headspaced your barrel's chamber.
I didn't know my reamer was a relatively bad design. I wouldn't have bought it if I had known that.
Your reamer will work. I think it has looser dimensions than ideal for a guy that's going to run an Improved version of any cartridge, because if you're going with an Improved version it infers to me you're looking to refine and maximize cartridge performance and accuracy.
I don't design and manufacture chamber reamers, but I've learned that the folks that try to improve upon factory chambers would reduce the "LEAD" diameter to 0.0005" over bullet diameter.
Not trying to be a jerk. I thought you were seeking advice prior to ordering your reamer. Perhaps your reamer supplier will exchange your's for a slightly different spec'd reamer. Or maybe you're happy with your current reamer.
About 6 months ago I researched reamer deigns for an Improved .338 Lapua fairly extensively. Below is the reamer I specified and ordered. Doesn't mean it's the world's best ever .338 Lapua Improved reamer design. But it does duplicate many of the reamer features and specifications I found other gunsmiths ordering.
There's nothing wrong with your reamer. As it sits you'll have a chamber which will allow you to run as a repeater and still offer a significant improvement over the standard lapua. Youll be able to simply shoot loaded 338 lapua ammo in the chamber to form brass if that's what you want to do. You won't need to neck turn brass, which is always annoying. And you own the reamer, so when you burn up this barrel you can have another cut to the exact same specs.
I've designed a few reamers of my own and done things like tight necks, extended leades, and custom leade angles. They made the reamer uniquely mine but they didn't make it significantly better than any others out there.
Chamber up a barrel, shoot it till the rifling is gone, and then do it again!