Are there any drawbacks to running a dual alternator setup with a 105amp and a bigger one on the other mount?

No. They both work toward a common goal, to bring system voltage up to the regulator cutoff. I can see no reason why it would matter if one was 200 amp and the other was 30 amp. If the batteries are low, they will both do their best to bring them up.

No its ok

What do the 105 and the 145(?) have different mounts?

I don't know that the 105 and 145 have different mounts. I assume they are all the same since they all can be used on the Dmax. I was just posing this question because on the gmdieseltech site he reccomends on the dual alt kits that you stick with the same amperage as the other alternator.

I have a stock 105 amp and a Powermaster 200 amp running for about a year now with no problem's

It has been done a good bit in the car audio seen. I can't see how it could hurt, but I'm not a tech.

I have had the same as DD for several years - works great!

The windings and armature are the only thing that is different between the two. The only possible problem (if there is one), is the different loads on the belt... the higher amp alternator would put an un-even load on the other accessories... That's just a guess.

Thanks for the info guys!

I have been running an OEM and a 200 amp (Premier welder) for the last year with zero problems. I monitored the output voltage and amperage of each when I did the install and they behaved appropriately.