Micheal,
please visit Andy Bright web site
www.digiscoped.com he has an excelent review of the sigma 500/4.5 and many high quality images taken with it.Also ask on bird forum, nikon and canon forums for thoughts on this lens, it is widely used by many.
Sharpness like beauty is in the eye of the beholder, only you can decide that.
F2.8 or 4 ,step up the iso !
See nikon forum on
www.birdforum.com for remarks on the 300/4 with converters, this should help you.
regards Subzero.