mail order may be cheaper but will be a generic tune.. on the dyno, it will be a custom one generally speaking (i've heard of a few places just putting generic tunes in)
Give Dick at Dick's Electronics in Sydney a call. Top bloke and knows his stuff. For the LS1's he does both mail order RTLS1 Mafless tunes and also live time mafless whilst driving. He'd be better to explain all this to you though Cheers.
definatly go dyno...mine was 225 standard & after a mafless dyno its now 312, as ashbox said it will be a custom tune via dyno witch is what you want should you have other mods