Yes and yes.
You definitely fill out underwear / posing trunks would also be filled out better... Assuming they are on roids, the silicone would likely help fill back out the sac (since the boys usually shrink a bit when on test).
If you haven\'t already, look up the member mrmantj here, he has some pics of his results both at 50cc and 100cc.
I also have some pics in my progress thread @50cc in the sac... no bulge pics in underwear though.
For more (sometimes freakish) pics of silicone\'d sacs, check out tumblr... some guys take it to the extreme though.
siliconeobsessed (dot) tumblr.com (gay content warning)
bebulge (dot) tumblr.com/ (some gay content warning)^^ this guy also sells silicone / injecting material... I\'d caution this big time... I\'ve talked to a tumblr user who has ordered from them and I asked if it was in a sealed bottle with a label etc (I was curious if it was silikon1000)... he mentioned that it was just a non-descript bottle with no label or any indication of what it really was. I\'m assuming its fine considering all the tumblr guys seem to use this guy as their supplier, but that\'s putting A LOT of trust in someone you\'ve never met.