Bro, pls sent me your email, I grant you access to my skydive.

if u r still on xbmc frodo or xbmc gotham, consider moving to kodi helix. kodi is the new name of the app, xbmc will disappear.
latest version is kodi 14.0 helix beta2. more stable than the predecessors. works on q16, works on q5ii, so go ahead:
just take note if u hv xbmc now, installing kodi will cause migraton of the .xbmc folder (addons, settings) to .kodi. meaning u can then delete xbmc to save memory, about 1xx mb.

find kodi 14.0 helix beta2 here:
http://kodi.tv/download/

a great new app applicable to kodi only is this bbts.
u gotta love >1500 live tv and radio stations!
mostly sd, some 720p 2.0.
includes news, movies, concerts, uk, usa tv, natgeo people, starworld, sky sports, russian tv, syria tv, bangkok tv, etc., etc. all live.
http://www.youtube.com/watch?v=DDNUm12VgJQ

i hv q16 quad on 1.0.6 and q5ii dual on 1.0.8.

spmc is a fork of xbmc. i tried it on q5ii, it works but i found it unstable, crashing. so i reverted to xbmc 13.2 and now kodi 14.0 beta2.
xbmc 13.1, 13.2, kodi 14.0 beta2 are all stable on q5ii.

q16, xbmc 13.1 and 13.2 are not very stable, crashing. kodi 14.0 beta2 is stable for q16. kodi (but not xbmc) can take new addon bbts 1000 live tv channels. tested it quite a bit, it is a great addon to have, the best live tv addon there is - usa, uk, italian, french stations... plus concerts, news, documentaries, history channel, travel channel, food network, etc., etc.

i do not know if the differences in performance of xbmc, spmc and kodi are due to brand, model, dual/quad or firmware or something else but i would think q5iii quad should be the same as q16 quad for kodi as the specs r the same except for emmc size.

i do not know if spmc addons/settings will migrate to kodi like it does with xbmc when u install kodi (a folder named org.xbmc.xbmc will migrate and get renamed org.xbmc.kodi). kodi now seems to be giving cold shoulders and understandably so, to ex-xbmc spinoffs like xunity.
that is actually good so that u dun need to set up all over again; just remove xbmc (or spmc?) app when satisfied with kodi.

yr concern is about playback via xbmc lagging... this i can't comment as i dun do local hdd playback on xbmc (understand u do that for extras like logos, get subs) i always use native player which works very well incl iso/bdmv/dtshdma. for q16 native local playback, pressing menu button on remote gives a photoshop quality setup option where u can define yr own picture quality parameters.