Sathiya, its a good thought, but, you will be least interested in the interface on a mobile screen, but more of functionality. Even with other modules/features turned ON, database calls are made only when any of the activities/modules are added to the page, else not. 

Moodle still have to make calls for the core template from database, and these are not few. With moodle and 15000+ respondents at the same time, you're looking at not just one server, but a server farm with at least 3 public facing web servers & replicated database (this is how resource demanding moodle is for this many people). Just to give you idea, moodle installation brings in 250+ tables into database, so if we assume half of these 250 are for core and rest half for activities/functions, still we are left with 125 tables, now if you compare this to a for example WordPress, it has 12 tables in total. so fetching data from moodle (125 tables for core) and from a WordPress (12 tables) you can expect load on database servers. 

SavSoft MUST be optimised for mobile interface as well, if it's quiz only, I'll recommend looking into their enterprise solution which at the time of typing is on offer, they do installation on digital ocean, and you can get the application hosted on Bangalore datacenter for a quicker response time, and as this application is purposely build so you can expect the core to be lighter to serve 15K+ respondents in a 30 minute time frame with ease and without setting up a multi server setup.  The amount and time you'll spend setting up moodle for quiz on Amazon EC, you may end up spending 1/10th of that with them without worrying about IF the application can hold that many respondents, as they are developers, I am sure they have tested this many users and know before hand how many resources are required to handle this many users, 

Least you can do is talk to them see what they have to say about it, as it's local to your international boundaries so one phone call away. 

Last, but not least, do consider the end user as well, how many of them will be willing to install an app on their mobile to take the quiz? will their devices be up to the mark to install the mobile app? Do consider the steps involved in taking the survey. if it's via a mobile app, they will be required to install the app first then take the survey (OK it cuts down interface render time for sure), whilst via website (Like SavSoft, SurveyMonkey etc), all you have to do is provide them link to survey and they can start straight away for software to take quiz is already in their mobiles (the Browser).