we here also doing conversion PDF -> G3 tiffs with high and low res. From my experience the best tool you can have is Adobe PDF SDK, the only problem with it is its insane price. So we don't use it.
what works fine for us is ghostscript, last versions are pretty much robust and do render correctly majority of the pdfs. And we have quite a few of them coming during the day. In production conversion is done using the gsdll32.dll; but if you want to try it use the following command line:
gswin32c -dNOPAUSE -dBATCH -dMaxStripSize=8192 -sDEVICE=tiffg3 -r204x196 -dDITHERPPI=200 -sOutputFile=test.tif prefix.ps test.pdf
it would convert your PDF into the high res G3 TIFF. and prefix.ps code is here:
<< currentpagedevice /InputAttributes get
0 1 2 index length 1 sub {1 index exch undef } for
/InputAttributes exch dup 0 <</PageSize [0 0 612 1728]>> put
/Policies << /PageSize 3 >> >> setpagedevice
another thing about this sdk is that it's open source; you're getting both c and ps (postscript) source code for it. Also if you're going with another tool check what kind of an engine they have to power the pdf rendering, it could happen they are using gs for it; like for instance LeadTools does.
hope this helps, regards