Its up to you whether is it worth it to leave for 2 days. Put aside those scary stories of car disappearing and such, you will have to think about logistic to and fro Singapore and another trip the next day, whereas you can uber/grab within Singapore. The savings may be worth it if $$ is your main concern.
Do not have the misconception that just because the site sponsors are sponsoring this site, they will surely be expensive. No. So far I have tried 2 of the site sponsors (near my place, based in Ang Mo Kio) and their level of service and support surpass that of PML (personal opinion).
Price wise, the 2 site sponsors are (definitely) cheaper than PML although between them there are some differences (1 charges GST, the other doesn't). Both are open to you getting the spare parts yourself from stockist. Their prices are quite transparent whereby parts and labour are usually separately itemised.
The JB workshop normally do not allow, if its oil change and for common parts. I've heard that for most other parts, you have to negotiate with them whether can bring your own parts first. Their prices would be in 1 lum sum so not sure whether they will jack up the labour when you bring your own/Do note that spare parts are taxable items when bringing across the causeway, so be careful when you 'hide' them in your boot.
