Whether Angel found Spike before he got his soul or afterward wouldn't have mattered. If he had found him pre-soul, there would have been a fight, at which time Angel would have learned that Spike was a far better (or at least more confident) fighter than the last time that they had fought, and he just might have had his ass handed to him on a platter the way it happened later when they fought over the Cup Of Eternal Mountain Dew. Spike was going to get his soul by that time, and he would have been at least as determined to get it as he was to get the Cup. It would've been a bad fight, both men would have been pretty badly beaten up, but in the end, Angel wouldn't have been able to stop him.
If Angel had found Spike AFTER he got his soul, when he was still in his cellar dweller stage, he would definitely have staked him, or, possibly, reverted to his Angelus personality enough to engage in a lot of torture first. The fact that Spike would have been basically helpless by that time wouldn't have mattered a bit to him, nor would the fact that Spike had a soul by then, or even the free ticket for what he, himself, had been given in Season 3 for what he had been responsible for the year before. Angel doesn't give forgiveness nearly as readily as he accepts it. Just ask Wes.
If Angel DID stake Spike in that condition, with or without the torture, he would have been wise to hope and pray that Buffy never found out about it, or SHE would have handed him his ass on a platter. Buffy would be just as free with her forgiveness and sympathy, not to mention protection, for a vampire who tried to rape her as she would one who tried to destroy the world after terrorizing all of her friends and murdering the woman who her Watcher loved.
The bottom line is, if Angel had found out about what happened in "Seeing Red," and had tried to stick his nose into the affair, his ass would have ultimately wound up on a platter. Far better and less painful for him that he remained oblivious to the whole matter.