Of course any delay / setback for the IJA in Malaya - probably delays / removes the Battles of Bilin River and Sittang bridge altogether as these operations by the Japanese (Kicked off OTL 15th Jan) took place after the major fighting in Malaya had been conducted and it was obvious that they were winning - thus freeing up those Japanese divisions and more importantly the logistics necessary to support them to attack into Burma.
If the British/Commonwealth forces are reinforced with first tier troops, stronger, better led, better equipped (or even simply rolled less 1s) etc then those IJA forces, and more tellingly, the logistics used in Burma will have to be thrown into the battle for Malaya instead.
Delaying the conquest of Malaya will definitely slow up the Burma Invasion.
QUESTION: All the Singapore/Malaya holds/delays TLs show what the Brits could have done to stop the IJA. And they all correctly point out that the 25th Army's logistics was shit. But by my memory, they never point out what more could have been done on the Imperial Japanese side to respond to a tougher Malaya campaign. What more could/would have been diverted to assist the 25th in conquering Malaya against a stouter defense? By my limited research, the following elements might have been available to varying degrees (I'm sure politics and the plan in place would be huge in determining who could be added to the 25th):
15th Army: OTL - The 55th Division started its invasion of Burma around January 15. The 33rd Division started into Burma around February 6.
16th Army: OTL - The 2nd and 48th Divisions started the invasion of Java on February 28. Would Malaya and Singapore have been viewed as important enough to shift these divisions there first? Or was Java of such importance that they'd only get pulled over to a harder Malaya after taking Java?
56th Division: OTL they first went to Davao (PIs), then to the DEIs, were slated to help in Malaya, but got transferred via ship to Rangoon in March when they weren't needed in Malaya after all.
38th Division: OTL took Hong Kong and then got reassigned to the DEIs.
If there were any other "available" forces, I don't know them. I'm not bothering to identify any of the small SNLF type forces used in the DEIs.
As for how else to help save Burma, the first thing to remember is that Monsoon season starts in May. The Brits more or less only need to hold out until then.
Second, the Australian 6th and 7th Divisions. Churchill asked that the 6th be deployed to Burma, but the Australian Government said no. Even if the 6th (or 7th or both) were allowed to go to Burma, it is debatable whether they could have made it in time. OTL They didn't start embarking on ships in the Middle East Theater (where I think both earlier had participated in taking Syria) until the end of January. And they didn't make it back to Australia until mid March.
The IJA 55th captured the port city Moulmein on the south side of the Salween River on January 31. They crossed the Salween on February 10. They crossed the Bilin River on February 17. The Sittang Bridge was blown by the Brits on February 23, forcing much of the Indian 17th Division to cross the river without their equipment. The 55th and 33rd Divisions started to cross the Sittang on March 2. The Brits retreated out of Rangoon on March 7 and the IJA marched in on the 9th.
If either the 6th or 7th can leave a month earlier and be allowed to land in Rangoon, holding on a Sittang line becomes plausible; however far from definite. It then becomes a question of how well the Aussies can handle the Imperials' infiltration tactics in an environment vastly different than the one they just came from. They at least would have the 7th Armored Brigade, who did excellent work OTL, to perhaps act as a mobile counter attack force against infiltrations. Holding a couple weeks on the Sittang then allows the Chinese 5th and 6th Armies to show up and reinforce the line (FYI I think a Chinese army was about the size of a Division. And their Divisions were the size of a regiment or brigade). And some of the Chinese Divisions were really, really good. OTL the Chinese fought in some excellent delaying battles starting in mid to late March as the British retreated to northern Burma.
OTL, the taking of Rangoon vastly helped the IJA's crappy logistics in Burma. Denying them Rangoon puts a big crimp in how much force projection they can do. Without Rangoon, 2 Divisions is probably their max coming over from Siam. With Rangoon, they were able to support 4 divisions in the drives north up the Irrawady and the Sittang to Mandalay.