Best possible outcome is that when he proposes the idea to Jefferson Davis and Samuel Cooper, that it ends right then and there, and that Longstreet's Corps gets shipped off none the less to Eastern Mississippi to join the push to relieve Vicksburg. If Lee wants to dare a sortie north at that point even a Defeat is less devastating as it would keep the Union Army distracted from Virginia, and Longstreet is still going where he might actually do some good.
So instead of having it do something that might actually have some possibility of influencing the Confederacy's fate, we send it to be unused* under an inferior commander**, further from anything that could lead to a significant consequence in the unfolding of the war***?
If we're talking once the Campaign Begins, have Lee not give into his General's stupid Glory-driven urge for battle (The Shoes story is a lie that makes no sense, down to the fact a Confederate column had already looted the town days before), and falls back as Longstreet suggested, to the mountains and a better position.
So how long does he sit there while hoping Meade is either so hard pressed by the administration or so stupid that he'll attack regardless of circumstances?
Now that my venting against "Let's assume that OTL was the worst possible scenario." is out of my system:
IMO, the best case scenario is something where Lee can maul the AotP badly enough for the Union to scramble to make up for the damage done to the rather low manpower wise eastern theater - which means taking men away from other theaters, which probably means Rosecrans, which makes life a lot easier on Bragg, which . . . actually does the Confederacy some good.
Best chance of that takes Lee being in a different battlefield than either Gettyburg or Pipe Creek and ideally being reinforced significantly.
* Let's say you send it in the second week of May (when this discussion came up OTL). It won't arrive before the end of the month
at best. Without its trains, because shipping them would take too much time and too much rail capacity.
By that point, the situation sucks.
**: Whatever else you can say about Joe Johnston, his record in the field has fewer victories than Lee, and Lee succeeded at more of his campaign objectives than Johnston.
Men who died in the Gettysburg campaign died in a campaign that inflicted significant losses on the AotP as well. Men who died of disease in Mississippi's swamps while Johnston struggled to convince Davis 23,000 men isn't enough to face Grant died twiddling their thumbs and sniping.
***: I strongly recommend that anyone who hasn't read the thread already read what trice (a better read logistical and strategic student than me) notes in this thread on comparing operations in Tennessee and Missisippi to the eastern theater in the context of how the Confederacy might actually do something to the Union army anyone cares about:
http://civilwartalk.com/threads/confederate-strategy-in-may-june-1863-the-what-ifs.10013/
In brief, though, what is a victory at Vicksburg going to do for the Confederacy? Virtually nothing. Its already essentially cut in two, Tennessee is no better off than OTL, and you just reduced Lee's army to probably less than 50,000 men (Pickett's and Hood's full divisions would be over sixteen thousand veteran infantry, and Lee may or may not get the OTL reinforcements he requested for his offense, so that leaves him without the infantry brigades of Pettigrew and Davis, the cavalry brigades of Grumble Jones, Imboden, Jenkins, and two of Robertson's cavalry regiments) versus an Army of the Potomac that is twice his size or more. And hardly likely to remain idle all summer.
Ignoring any reinforcements Hooker gets, and measuring its strength as of mid-May.
This is not a gain to the Confederacy - even ignoring the supplies captured campaigning in the OTL Gettysburg campaign.
So what triumph do you see Johnston achieving that I'm missing?