Steven Attewell considers the same change to the timeline in his CBC analysis, Eddard XI. I'd recommend reading the whole thing, but the excerpt that deals with Loras goes:

What if...

Loras had gone? As I said, given that the Tyrells enter the war against the Lannisters anyway, it’s tricky to figure out how much actually changes.

One thing that does come to mind is that the Tyrells might be more inclined to view the Starks as allies, which might mean that Renly signs an alliance with Robb Stark on the spot when Catelyn meets him, and/or that the Tyrells reach out to Robb Stark rather than Tywin Lannister following Renly’s death.

After all, the Starks are a powerful House in control of two of the seven kingdoms, and Mace has sons and daughters and nieces and nephews who could marry into the Starks, so it’s not a bad idea from Mace’s perspective.

It would immediately change the war: even if his western offensive had been cut short as in OTL, Robb would have enough men to assault King’s Landing and wipe out the Lannisters, regardless of whether Tywin minus the Tyrells was enough to save the capitol from Stannis.

On the other hand, if Loras got captured at the Mummer’s Ford, the Tyrells might seek a Lannister alliance even more readily than in OTL.

On a hypothetical third hand, if he dies at the Mummer’s Ford, there’s no deal to be made at Tumbler’s Wells, and Tywin would have to save the capitol on his own, and even if he had succeeded would have had to face the Starks and the Tyrells together or separately, a much more difficult task.

However, in that scenario, there’d be a decent chance that Mace Tyrell would blame Lord Stark for getting his son killed, scotching any hope at an alliance.